I wrote a paper earlier about the problems associated with VFDs. The problems aren't the controllers, it's the installation and misapplications associated with them.

Instead of spending a lot of time explaining the advantages of them on fire pumps, you can rummage through everything here.
http://www.mastercontrols.com/Variable_Speed_General.html#Variable_Speed_General

They offer a unique advantage of constant pressure output on city mains with wide pressure swings for 20 - 50, 60 PSI, utilizing the 20 PSI minimum without overpressure at 50 - 60 PSI. This eliminates high-rise valves and fittings on lower floors or PRVs. They offer the same advantage with extremely tall, narrow storage tanks, utilizing the lower levels for suction design. Sprinkler designers will probably find it a big savings on material and labor on the heavier and more expensive fittings without one in certain locations depending on the pumps churn pressure at the city's normal pressure..

In Houston Texas, we use small 2500 GPM break tanks with complete refill at 150% so they don't offer the hydraulic advantages like in the aforementioned situations.