My Mommy/GF was just talking about this the other day and she said that it seems terribly lazy and impractical, I believe her exact words were along the lines of, "do you really need a siren or a text to know when a diaper needs changing? I mean, it's rather obvious when they need changing, isn't it"
After some thought though, I think you could look at this in one of two ways; domestic use and job use.
For domestic use, I agree with the OP and my Mommy, having a sensor to inform you that a diaper needs changing reeks of laziness. A diaper check or a quick sniff test is really all you need and it's all parents have needed since what, time immemorial or at least since cloth diapering became a thing
Also, I believe that mass producing diapers with sensors would likely drive up the price point as well, this makes the idea of an electrical sensor in diapers both frivolous and costly
If you think of this for business/service sector use, especially in hospitals, daycares and nursing homes, then I think it is a solid idea. I've heard horror stories that at some hospitals, patients are changes infrequently because the nurses on staff usually stick to their standard rotations, leading to complaints being filed. I think under certain circumstances and in certain spaces, a sensor in a diaper could ensure that the individual is changed in a timely manner. Actually, this is really the only practical use I see for diapers with sensors, as it would save time and complaints and it would eliminate a lot of guess work as to who needs changing in a daycare or nursing home environment