No, emulation is simply more than just the board for arcade machines.
It's as accurately representing the actual game state as much as possible. Thus why things such as MAMEHooker are supported, such as they are, by the MAME devs.
Some games actually REQUIRE the artwork to be functional, as well-- Spy Hunter has five separate lit pieces of artwork detailing car status, for instance.
On top of that, some games may have been designed to take into account glass curvature or other effects of the beam, making HLSL the choice for getting more accurate output.
Try checking the MAME manual at http://docs.mamedev.org