Well it does hide the bezel and is part of the cabinet style on some games, like Crystal Castles and Missile Command. They wouldn't look "right" without the tint. On the other hand, Dig Dug or Centipede would look strange with an extra layer of tint under the glass bezel.
On the other hand, a LOT of games don't have tint, especially after around 1990 when the tubes started having the tint right on them. Notice how a lot of early tubes look light grey when turned off? Most new tubes already look dark when off. If you add tinted plexi to one of those, you're just cutting out more light and not getting any real benefit. Most of my 25" monitors clearly already have tint on them, and a lot of 1990+ 25" games don't have tint on the glass. (My 91 Virtua Racing's do have tint, my 92 MK and 98 Daytona 2 do not).
My Mspac cocktail has a newer monitor and no tint, and honestly I think very few people would notice the lack of tint because the monitor has plenty of contrast already.
I would lean towards what was original, with the exception of having a NICE monitor, I might remove the tint if it was no longer needed or was damaged.
Wade