Just some observations based on what the list shows:
1. There needs to be better standardization amoung the output names in mame drivers. Most are pretty clean, but the main offender is the solenoid. In sidewinder they are properly named, but in qbert and other classics they are "knockers" and in others "kickers". In one of the games they are even referred to as "kickbacks".
2. The question arises if using a good description of the given output in mame is a good idea. Right now the list is farily standardized, but you have no way of knowing what is what, other than what kind of output it is. On the otherhand, look at sea_wolf for an example of how it'd look if it weren't done that way.
3. Because the output system is so closely tied to the artwork system, there are several outputs we can ignore..... "overlay" and "backlight" are purely artwork releated, and wouldn't be useful for anything else. Also game systems notorious for garbage on the screen have a "blank" output, which Im guessing blanks out the garbage, I dunno.
4. As a whole, not many games are using the output system for anything interesting. Most are neogeo counters, start lights, and artwork garbage.