Unless you have it hooked to an alarm or control circuit that can shut down your system in case of pump failure, a flow meter is about as useful as tits on a bull.
on modern custom water loops, pump failure wouldn't even be enough to CPU throttle @ idle
I can kill fans and pimp and the box will continue to idle on.
I've found that when a pump dies, the computer throttles and or locks up within about 5 seconds.. there's really no time to shut down, so I find a flow meter unnecessary.