Yeah, those older titles would fall under the banner too. I think the positive is that with a service like this, SEGA would have to release a steady stream of rereleases to keep atop the release list. As it's a defined sub-category rather than discount Genesis games falling into the pool of XBLA titles.
I agree, Saturn games rather than more Genny/Master System titles, however I welcome any online multiplayer Genesis/Master System games. Online 2 player co-op for TJ&E would be great.