A theme switch module which allowed you to choose theme engines/themes based on the view being generated may be fun, but how often on a site do you really need this level of abstraction in the presentation layer?
lets get pragmatic here. sometimes users want a different look for their "space". "different look" might meen customized blocks and customized colors. for example, the Brazil organic group might want the Soccer News feed on top and the yellow color scheme. their organic group. is that a valid requirement. if so, how should it be implemented? it is fine to say that most sites/modules not switch the theme. thats accurate. but sometimes it is a valid system requirement IMO. alan's link points to a bootstrap bug thats been fixed. i don't think it strengthens anyone's argument unless you believe in folklore like "bad things happen when you switch themes" -moshe