This is a big signing - Wallace is getting older but could still be a starter, and is choosing, if he's to be believed, to play in a team atmosphere because he wants a ring.
He's a player that might not be on the floor to start a game but could well be on the floor to finish - especially when we need some strong defense - and he's got the size to give Pierce or KG a break without giving up much on defense, and can still score when he needs to.
I don't think this changes anything in terms of the teams pursuit of Grant Hill, another verteran backup with the diversity to give Pierce a break or can play SG - and Hill was healthy all last season too.
Seems to me this team is being built to weather potential injuries (though the Championship still hinges on good health for KG and Pierce come the playoffs) but also to ensure strong defense and fresh bodies late in the game - all of these things were weaknesses of this year's team.