I'm still rather new at this, but this is my understanding. Somebody correct me if I'm wrong, but the general concensus seem to be that it slows down play. So from a gameplay perspective you probably want to get rid of it.
As for increasing or decreasing value, it depends on the type and condition of the playfield. A mylared clearcoated playfield will look just like new after you remove the mylar. And removing it normally won't cause any damage. In the early days of clearcoating, some playfields were still mylared by the operator because "that's what you were supposed to do." At an auction I saw a Diner with a Diamond Plate playfield that was completely mylared. Once that mylar was removed, the playfield would have looked brand new. And yes, it really was a Diner with a Diamond Plate playfield.
With the pre-1990 painted playfields I think it is more of a crap shoot. Removing it could take the artwork with it, especially if the playfield has seen a lot of use. So you have to proceed with caution. It is also tougher to remove without doing damage.