To charge Hobbs time is probably a rip-off, and leads to pilot frustration.
At today's prices, any time spent taxying or (especially somewhere like Biggin on a fine weekend) waiting in a queue to get aloft is wasted, expensive time.
And with the engine at, say, 1200 rpm, and no stresses acting on the airframe due to flight, the costs to the operater are simply not those on an aircraft in flight.
Time between maintenance is also not based on Hobbs time, but upon flying time.
It doesnt't seem to be value for the customer to charge on Hobbs in a rental scenario.
If it's how a group wants to operate, where one is looking at costs rather than margins, that may be different.