He might be wanting to have a WSB championship before making the jump. This year it could happen, but if not he's very likely to get it next year. MotoGP, OTOH, he knows is still a different game, with a steep learning curve, and that he'll be a little while catching up.
Another possibility is that the "rookie rule" requires a certain number of MotoGP races before moving up, not a "season" specifically. Spies has already done a few as a wildcard with Suzuki, so if they can put him on a bike for nine MotoGP races next season, on the side, no pressure to perform, he qualifies as a "contracted ride" to skip the satellite and go right to the factory team in 2011. Or maybe they are planning to try to get the rookie rule dropped.
PhilB