Bottom line is, it's better to have something than not having something (For all you wise arse's out there, I'm talking about SM3.0 full compliance and not herpes. ) and wishing you did when the situation arises.
If you think you (whomever you are) can deal with a card that does not support the latest technologies, then go for it. And don't get frustrated as time goes by when more and more titles emerge and there is another thing your card can't do. And then another, and another. Even if you (whomever you are) don't think that this will happen, why force yourself to buy a lesser card? It makes absolutely no sense to buy a card (X800/50 whatever) and expect it to be able to do everything a card with more features can do (video processor aside) forever. Riddick, Splinter Cell 3, FarCry HDR is just the tip of the iceburg. I can't prove to you that I'm right, but you can't prove I'm wrong either. But I would rather be prepared than not. And I am. 6800GTOC. My ASUS X800XTPE sits on the shelf, I sh!t you not. I tried selling it at a reasonable price in FS/FT, but nobody wants it.
To the OP. Your original question, "Why go with SM3.0 today?" is easy to answer. Had you phrased it this way, "Why not go with SM3.0 today?", nobody can give a valid reason. And the answer of, "Because you don't need SM3.0", is not valid so pay no heed. Not everybody "needs" a kickass video card let alone "needs" to play games on a computer.
But we want these things don't we.... ALL of Nv's 6 series cards are SM3.0 compliant. No matter which one you choose. from the lowly 6200TC to SLI'd 6800Ultra Extreme's. Pick and choose your price range, there's plenty to choose from.
The answer to your original question is another question. "Why not?".