Because the performance increase can actually make a difference.

No AA:

http://img413.imageshack.us/img413/8195/crysis0xaawq8.png 

2 or 3 frames is pretty dang nice I'd say.  AA is needed to see the 1.2 patch come into effect.

AA enabled:

http://img413.imageshack.us/img413/2362/crysis2xaavv4.png

The article can be found here:

http://lly316.blogspot.com/2008/04/crysis-performance-benchmarks-10-vs-11.html 

Make sure to patch before the next time you boot up Crysis, guys.  Definitely worth it.

i wonder how many other games this holds true for and great post