Most of the figures in my collection are cast-offs but that's really beside the point. I just find many of the 18+ figures released are very well sculpted and incredibly elegant and cute, especially neko girls .
I pretty much always display them with their clothes on but because the outfits are very well done and feel like I'm casting-off much of the figure's appeal when I remove their clothes.
Generally, I only do so when taking photos, both with and w/o for information purpose to other MFC members who might be interested in purchasing the figure. Though I sometimes come across a nice beauty shot in the buff.
I don't think there's anything wrong with purchasing 18+ figures for male and female adults alike, if you enjoy them on your display. That being said, I'm still courteous to guests and don't display them openly in the living room. I feel it's best to keep them in the bedroom or basement, where people you know who are cool with it can be offered a tour.
Some people collect comics, some guns, some cards, porn, etc. These are all acceptable hobbies. I don't see why figure collecting shouldn't be. It's not as common as the others but anime figure collecting is growing more into the norm in the west.