The V's are an improvement over the III/IV, but on sheer performance, no synthesized unit is going to outperform a crystal controlled one. The V's still have their share of issues, such as the tendency to false alert. It was enough of an issue that Motorola had to include in the software a "Falsing Alert Adjustment" screen whereby you were able to decrease the decode sensitivity. The drawback to that is the pager is less likely to alert properly in fringe reception areas. By comparison, the II would often still alert even when the RF wasn't enough to open squelch.
The III and IV series were junk. Made in Taiwan, and not by Motorola. In fact, save for a few external parts like the knobs, they were identical inside. There are many IV's out there that, if you open them up, have circuit boards with "Minitor III" printed on them. Both models had a well documented problem with the circuit board flexing, which would detach the decoder IC and the pager would thus not alert.
I do agree that it's quite impractical for an agency to keep gear around that hasn't been supported in years and to which parts are just about impossible to find anymore. However for personal use, you can find some great deals on used units that have many years of service left in them.
All in all, I feel that because the II is a better performing unit than the later released models, and can be had relatively cheap, it remains a viable option for someone looking for a reliable pager.