The airframe and engine log books are required to note daily the total time airborne and the number of flights. I think this is the only legal time recording requirement for a simple single piston. A Hobbs is fairly useless for this unless it's got a link to airspeed or undercarriage. However, industry practice is to note the Hobbs time and subtract 5 mins from each end of the flight. (or 6 mins if you're a purely decimal organization).
As you say, the Hobbs encourages not only rushed checks but 'firewalling' the throttle during flight. However, it is usually the only way of accurately recording elapsed time so most people rub along with it. I can't imagine any aircraft operator taking the whole of the Hobbs time as aircraft log book time for maintenance purposes. The difference SHOULD be around 20%.
At one place I worked, we used Tacho time. Over a period, it trued out the same as airborne time and was accepted by the CAA as an equivalence. The big plus with this is that it encourages people to use sensible airborne power settings and on the ground, any delay at the hold is less expensive.
What's needed is a reliable airborne time recording system. Many GPS offer this facility. This still doesn't get round the problem of hirers firewalling the throttle, though.
Cheers,
TOO