Another reason for using studs is that, for several maintenance cycles, they can stay put when dismantling, prolonging the life of the tapped holes in the expensive bits.
Hmm - that was certainly the other way around in my youth ...!
Most moped cylinders had studs for the inlet and exhaust and the thread on the exhaust ones where always bad
So changing them for bolts always solved the problem (if we could get them out - else cylinder = scrap) - the same with full size bikes, so we changed them to bolts too and never had any problems after (unless some jerk tightened them too much - but Heli-Coils cures that).
I would NEVER use studs in anything I design - but most here are building copies of old things and that is a different matter.