I think a hardtop increases the value of a machine it's installed in.
The original manufacturers spent a lot of time and money to make their playfields smooth and shiny with great artwork to provide the player with a visual treat while ensuring fast, and zippy game play.
Unless the original playfield is in perfect as-new condition a new hardtop will make a tired playfield new again. Thus increasing it's value.
Original playfields are awesome... when they're in as-new condition, and 99.9% of what's out there isn't anymore.
-Paul