Plenty of CAR audio can be used in home theater and have nice flat response curves actually, if you know how to design proper boxes.
3/4 inch for BOOKSHELVES will always be overkill unless you have put in a dedicated low frequency driver which usually is only done with TOWERS(in which case yeah 3/4 inch is great to be using)
Especially when using under 20 watts of power, 20 watts isn't going to be bending too much 5/8 or 1/2 inch bookshelves they're small enough to need minimal bracing on the side panels.
A experience DIY would know, yes you can make speakers and sub woofers with small walls and bracing. But if you are looking for a good sounding, high end, sub woofer or speaker, A heavily braced, and thick box is needed if cost is no issue.
Edited by LukeG - 1/22/11 at 11:33am