What you wear or don't wear is up to you, as long as you are respectful of others.
If you want to wear diapers regardless of the fact you don't need them, there is no real reason why you should not as long as it is not impacting others in a negative way. If you are discreet and keep them to yourself, changing in a restroom and not in the shared room. You should certainly make sure not to go #2 in them or allowing other stagnant diaper smells to linger around that would be offensive or impacting the experience of your roommate.
It really is about respect. Think about how the other person you may be with might feel if they are exposed to these kinds of things. Your desires for wearing should not infringe upon another's rights. If you are not incontinent, then yes, you have a choice. And while wearing a diaper in itself is not something that is infringing upon the rights of your roommate, actually using them in a manner that they have to smell unpleasant things that certainly does.
If you are courteous and considerate of others in your wearing of diapers, then there is no reason why you shouldn't do so. Just don't flaunt it, and be as discrete as you can. If the question comes up, then share that you are wearing. But as for the explaining the reason, I guess that is partially based on the situation. If you and your roommate become close friends where you feel you can share the real reason for doing so, then go ahead and explain. But if not, you don't really need to explain that they are not physically needed, but just admit that you wear them and will try to be as discrete as you can about it. Then act in the same manner. Be discrete and keep it to yourself.