The chemical plants where I worked were full of them, even on small motors. They paid for themselves in power savings, control, and motor sizing.
I would estimate more than 90% of the pumps and fans in chemical plants, pipeline pump/compressor stations and refineries do not have VFD's.
They're wasting a lot of power.
It depends on the application. When you have a cooling fan bank of 20 fans, they turn on/off each individual fan.
Smaller banks, say 2~4 may have one VFD, switchable between more than one fan.
Motors can have a much lower efficiency outside their rated design point. It is be more cost effective in some cases to break up the service into multiple pumps. This also helps in a N+1 type redundancy that allows individual motor/service to be taken out of service while doing maintenance and keeping production up.
Most refinery service is designed for a specific flow rate. No only the pumps, but the separators, piping, reactors, etc all run at a set, non-varying rate for many services. There is not a need to vary, but a need to design the proper size head vs flow in the impeller and it runs there for a decade.