I know this is a zombie thread but I've had lively discussions with two Orlando FSDO inspectors claiming this and I took them to the cleaners with the FAR-AIM.
On my phone right now so can't look it up but FAA council has also ruled on this.
Hobbs time is an FAA accepted way of logging flight time.
So when the Hobbs runs you are authorized to log it.
They also tried to tell me you couldn't do maneuvers during a cross country.
Eg...a 100 mile XC and you slow boat it at 90kts instead of 120 and you do ground ref maneuvers on the way.
Ends up being a two hour flight instead of .9
That's all XC buddy, and legally so.
The Orlando FSDO suffers from a severe case of "tribal law".