Miller would strictly be an off the bench player. In the late goings of a close game, you can put him on the court on offense and call a time out before the change in posession (he might be a defensive liability). But having him come off the bench for, let's say 8 to 12 minutes would be a huge asset. But if the coach decides to go with a smaller team on offense, late in a game; with KG down low and Allen, Miller and Pierce up top the D would still have to guard Miller posted down low on the corners (where he likes to shoot his 3's from) -that would give Allen his more favored 3-point shooting area at the sides off the top and it would allow Pierce more room to manouver if he decides to take it to the rack.
If and this is a big if -if Miller comes back; he's a pro, and he would certainly, at 42; recognize his role on a team like this. He's never been the type of player that demanded the rock but he has no qualms of raining it down when the team needs it.
