No, of course not. Window compositing is a huge usability feature. It made me switch from xmonad to Gnome 3, because it just feels so much faster and responsive. Also, live miniatures are almost necessary (at least for me) while dealing with 5 terminal windows and another 6 emacs frames. And you lose nothing (well, ok, you do, in some cases it can be slower, take few more resources and so on, but that's the case with only some apps, not general). Making compositing and using accelerated video HW the norm is a step forward, not an unhealthy trend.