Thome will most likely serve as a fill-in, should Ryan Howard not be ready by April. Once Howard returns, Thome should be a solid pinch-hit option (15 HR, 50 RBI last season). Quite frankly, it's a move I wish the Phillies would have made this summer, because they certainly could have used somebody to pose a threat off the bench against the World Champion (that hurts) Cardinals.
However, Thome is a slow power hitter who strikes out a lot (always has). He's also only played the field once since 2007. That's not exactly the type of player the Phillies need– it's the type of player they don't need more of.
The real upside of this move is bringing in a threat off the bench.
Either way, it's great to have him back.