For a shop having only single-phase mains, having an RPC to drive constant speed machines is a fine thing.
Having VFDs on variable spindles is a wonderful thing.
They need not be used together in order to reap the benefits of either.
IF you're powering a spindle off a VFD, then just use a VFD that will accept single-phase input.
If you have other machines that don't benefit from variable speed operation, then hook 'em to the RPC, and only start the RPC when you need those tools.
I have both. I ran a home-built 10hp rotary for 20 years, and over the course, installed VFDs on all the spindles of the mills, drills, and lathes. I even installed them on odd applications including, but not limited to table saws, exhaust fans, air compressors, and concrete power trowels. They have their drawbacks, but in terms of operational performance, if they're not performing well, the usual issue is improper programming or an inappropriate selection of drive components and drive ratio.
The simplest VFD conversion I have, is on my J-head bridgport knee mill. I removed the direction switch, wired the VFD direct to the motor, hung the electronics on the side of the column and put a control pendant on a hook on the front of the table. I still use the original V-belt step pulley, but it's not for any particular advantage aside from the fact that it still works fine. I have the ability to use it to change drive ratios, and the J-head backgear still works, but the belt is more or less parked on the center sheaves (1:1 ratio) because that point is where belt contact maximum occurs (any ratio other side yields a smaller contact on either the motor or the spindle). I mill everything from wax to steel, and have no problem with operational limitations. The only thing I DIDN'T do, which I probably should have, is put a constant-speed cooling fan on the motor, so that it'd cool well at substantially slow speed, but I haven't experienced any heating issues with the big pancake motor.