Programs being launched faster and things generally starting faster is the main advantage. On my brother's ASUS Zenbook UX51VZ (with 8GB RAM, an i7 quad-core, and a 256GB SSD) GNOME shell starts in about two seconds. When I installed GNOME shell on my computer (an ASUS N43SL with 4GB RAM, an i5 quad-core, and a 500GB HDD) to try it out, it took over a minute to load. The thing is, there's no significant difference between the two computers in terms of processing things (the i7 isn't that much faster than the i5) and they both have enough RAM. There is a massively noticeable difference between the two in writing and reading from disk. On his computer, programs launch instantaneously 99% of the time (even when running GNOME!). On mine, programs usually take a few seconds to start (even when running Xfce). Even Xfce takes about 20 seconds to start up.
Needless to say, I use Xfce.
