We will see how it all plays out, I actually have never been a huge fan of back catalog games, its always seemed more like a marketing bullet point to me. I think people hear "thousands of downloadable games" before they remember they only really loved 5-10 (if that many) per particular system.
The battle for second console in my house will be interesting, there are some older playstation games that I really liked (I loved Loaded and Re-Loaded), having never owned a PS2, there is that entire catalog. But I also just bought 2 DS's and the whole house loves them, and N will have the back cat of Sega and TurboGFX, not to mention their prior consoles. It will be interesting; for the moment though, I still rank XBLA above that of back cat only methods, if only because instead of JUST old games I get some new stuff as well, and the new stuff is just as fun, AND prettier.