Fleet-wide upgrades are like a snake eating a pig. It's a huge project that draws resources away from more routine support activities. Piecemeal upgrades can be done as part of the normal replacement cycle.

Field deployment can reveal issues that remain despite testing. Fleet-wide deployment means everyone has these problems and they all have to be fixed at once.. Piecemeal means only a few do; once they're dealt with, there's plenty of time to fix the issue for the next system.