Writing service files for my own daemons or modifying existing ones is pretty close to trivial. The files are short, easy to understand, and there isn't any risk of runaway child processes like there is with a sysvinit init script making them close to trivial to write and maintain. If anything I would say that's why so many distros are jumping on board.
I had to write service files as an early adopter, but it would also be useful for anyone rolling out their own daemons or that needed to tweak the behaviour of an existing service for their own needs. I imagine it would also lead to fewer packaging bugs.