To turn that around, the PV element could be used to drive the pumps, thus making it a completely self-sufficient system and not subject to (say) power cuts.
If I still do solar thermal, and I'm humming and hawing for a number of reasons, the self-contained nature of such a system would be a good thing in my mind.
Mind you, I already have loads of PV on and off grid, so I should probably quit whining and just use that to power SDHW in a once-in-a-blue-moon grid power cut...
Rgds
Damon