If I decide to wear the cami that comes with a sheer-ish blouse, I always snip those little loops that hold it in place. I think the loops are more meant to keep the cami and blouse together as a 'set' in the store, more than it is to keep the cami 'in place' on the wearer.
Sometimes I like to wear a different cami with the blouse too. For instance, I have a green/blue-ish paisley-type blouse that's sort of sheer, and it came with a royal blue cami - which I'll wear sometimes, but then I also like my navy cami and my green cami with this same blouse. If I left the blue cami 'attached', I wouldn't be able to do this. And besides, I can't imagine trying to launder the two if they were still attached...I'm sure it would drive me crazy! ETA: it already drives me crazy trying to try on these types of 'sets' (cami/blouse combos) in the dressing room. No way am I going to fight with that.
Hope this helps
PS - ETA again: I also like to make a point of making sure that any cami that comes as a set with a sheer-ish blouse has adjustable straps. My Mom has bought blouses that came with a matching cami, only to find that the coverage wasn't where she wanted it.