Howard isn't exactly a good defensive first baseman either.
There is a really good chance they are going to get the same level of production from Thome only he's not going to be making $20 million to do it. It's a really nice pickup for the Phillies.
Even if Thome literally can't play 1B then he's still a great pickup for a guy off the bench. I'd imagine the plan is to have him platoon with Mayberry somewhat though, which is probably better production than Howard alone.