Well, at least my attempt at a definitive answer.
Sometimes we forget that the unknown and uncontrolled elements in the world, our lives and ourselves are simply facts of our reality. As a result, we don't know how to face or deal with them, and this fills us with anxiety and fear. Thus, in order to ignore them, we label these things as absurd, strange and sometimes even bad. However, this is only an excuse. In truth, since they are unknown and uncontrolled, the goodness or badness of their results is also unknown.
Now, the probability that they will have one outcome or another may be understood to some degree, but to be certain, we must actually experience these elements. Doing so will give us some additional perspective and knowledge of them, thereby improving the outcome of future interactions, and eventually, making the choice obvious. By definition, this is good, and hence, we find that the unknown has a bias towards good.
Of course, there are also uncontrolled things for which we lack enough information to make a clear judgement. For them, the only certainty is that by experiencing it, we will come to know it better, and this is good. Therefore, such unknowns must be embraced headlong with verve and excitement, for even if it has a negative result, we are all the more improved and complete by knowing that.
In this sense, diaper wearing is a profound threefold metaphor:
First, diapers are associated with the child, whose only certainty is the near constant confrontation of the unknown. Yet, somehow, they are not wholly overwhelmed by this fact and their anxieties are few. Indeed, they instinctively realize that the unknown is, if accepted, an opportunity to gain knowledge. This is an exciting prospect, and thus, we see the essence of innocence and child-like curiosity. Moreover, this is something anyone can have, if they allow themselves the privilege, and that is a comforting thought.
Second, diapers exist to deal with incontinence, which is a very fundamental unknown. We are all born in this state of being unaware when our body will expel its waste. Even so, this uncontrolled element is not terrifying, for we have mechanisms of dealing with it, namely, the diaper. However, these cannot be utilized if the incontinence is denied, which would be a very foolish thing to do. Instead, one must accept the lack of control in one's life, and through this, regain it.
Certainly, it can be said that those with the most control are also capable of losing it. Consider the astronaut in spacewalk, where every minute is carefully co-ordinated, extremely expensive, and potentially dangerous. It would be ridiculous to spend this incredibly valuable time on something relatively unproductive like going to the bathroom. Thus, they wear diapers. Again, imagine the entrepreneur or coup d'etat leader, who after the initial wave of success believes they can carry on forever. In fact, this requires a different skill set; one they may not possess. Thus, for their dream to survive, they must often give up their power. It's clear enough, then, that sometimes giving up control is the only reasonable option, so one must always be able to do this.
Finally, wearing a diaper is itself an admission of the unknown. After all, society at large is often uncomfortable with the practise, whatever the reason for it. It's known that many squeam at the thought of baby changing, joke about Depends and insult the bed wetter. Certainly, when you wear a diaper, there is an undeniable awareness that you don't really know if others can tell, and if they can, you don't know their thoughts. This is even more potent when purchasing them and in close relationships, and then there are leaks! Even when alone, one wonders what others would think if they knew. Moreover, most of these things are very difficult to determine, so they will remain unknown. All in all, it takes more then nerves of steel to wear a diaper: One must truly accept the fact that the unknown exists in every facet of life.
Although in it, one meets many people who erroneously believe that they can and ought to have absolute control in some regard. However, it can be observed that over time they will succumb to various neurotic behaviours regardless. After all, believing in falsehood is delusion, and doing so makes one prone to further delusion, that is, neurosis. In the end, this is but a different kind of incontinence, and these people become that which they fear so much. Unfortunately, this destructive misconception is very common, so one must work that much harder to avoid it, but even then, there are still unknowns.
As such, we all encounter the deluded one way or another, and most of us will be negatively affected by this. Due to common male social expectations and the parent's importance, this will most likely come from a domineering father. Although, certainly bullying, other abuse and dramatic social change are possible causes among many. In any case, we might forget how to handle the unknown, and become incomplete as a result. If this resurfaces, then we will naturally search for what's missing, even if we don't realize it. Depending on our situation and experiences, we may come to any number of conclusions, including becoming deluded ourselves.
From this understanding, the basis for diaper wearing by those with bladder control, adult babies and other forms of infantilism becomes much clearer. As it has been seen, these things tie in very strongly with anxiety, the need for control, the unknown, and the acceptance of one's situation. More importantly, they remind us of our childhood and innocence, in which such problems didn't exist. In other words, they provide a solid realization that it's OK for unknowns, differences and uncontrolled factors to exist. Also, this need for purity is perhaps why infantilism often comes with a strong appreciation of fantasy, cartoons, anthropomorphism and so on, because of the abstract purity and innocence often found in their theme and structure. Overall, if our internal struggle has to do with these issues, and we have experiences that connect with it, then infantilism may be the solution we find, and hence, demand of ourselves, so that we may remember what was once forgotten.
This leaves one final question, however, should one actually partake in it and to what degree? First of all, we must look at what we know. Namely, if it were widely known then there would absolutely be negative responses from a significant number of people, most would consider it very weird, and the rest would either not care or be receptive in varying amounts. Although, with care it can be very discreet. In fact, it needn't be done in public at all. Also, there is the necessary expense of time and money, but this can again be minimized with planning and the use of cloth diapers. Of course, this evaluation is also highly dependant on one's situation. Some might be institutionalized or even killed by their social group, while others might be embraced by it. However, for most cases it's very grey and unknown, so as logic dictates, it ought to be tried at least.