Yea....SLI is almost never worth it except for 2 things.
1. You want the most bleeding edge technology you can get ( SLI 8800GTX)
2. You Buy one say DX10 8800GTS for only 300 bucks becuase you dont have enough to shell out for the GTX, then you add another later on for more power.
Still though SLI is almost always a waste as it makes more heat, requires more power, and becomes obsolete pretty fast.