It really can dampen the mood when you come out of the shower or just sort the financial side of things, and suddenly the lovely outfit or sexy lingerie is suddenly off, and you don't get to have any fun with them on. It takes the fantasy out of the encounter, and makes it all feel rather rushed.
I do prefer to undress the lady myself, or to keep some of the ooutfit on, if it's a uniform-- I once kept the skirt on a lady when she dressed as a school girl for me, and kept the 'naughty nurse' hat on another-- great times!