I got mine basically because I could afford it also. I had caved in from buying a new AGP card to doing a complete rebuild. I love my SLI it runs everything real smooth, much smoother than only one of the cards did. (I initially bought only one, but liked the boost so much I got another)
If and only if you have the money to burn go for it. Otherwise in most cases a good single card is good enough. 😀