I've been running one on my Mac 32 for a couple years because the G230 (Car engine) carb I used (it was handy and a good match) had one, it is handy as heck! Like you said, flushes the carb, pushes out air for the first start if it's been a while since the last time you had it out. If you run a 3 line fuel system, tee the fill line into the return line from the bulb and even if there is an air leak in that line it doesn't matter at all.

If it wasn't physically on the carb I don't know if I would put one on, but I've thought about doing it, and I think if a plane I put together has a carb that's set up for it I might try to incorporate one... Might just be another one of those conversation starters like explaining to someone that your engine used to cut weeds or wood

One down side on the carb mounted bulbs I've ran into, if there's no fuel in the bulb they don't seem to want to draw fuel if you forget to prime the system, I think it might be from the membrane or whatever is in it's valve assembly.