Look:
I've been around this business for 48 years and have well over 24,000 hours. I have flown and taught professional grade EFIS from Collins to Honeywell since 1984.
I am saying that maintenance on an aircraft is based upon Time In Service. Time In Service is defined as when the aircraft leaves the ground until it alights back on earth. If I taxi out for takeoff, but return to the ramp and shut down, there is no Time In Service. None. Zippo. Nadda.
There is no definition in the regulations for tach time.
Tach time is something we used to get with a recording tach. It was better than the oil pressure activated hobbs time for maintenance because it is generally less than hobbs time since engines average less than the rpm's that equal one hour of operation. Many LSA's equipped with Rotax engines do not have recording tach's.
Besides all that, I want to set up a maintenance schedule based upon what the regulations require. If it appears that the schedule is not enough, then I will decrease the intervals (still based upon flight time).
A simple trip log will solve the problem: Out/Off/On/In. In-Out will give the pilot his Flight Time. On-Off will give Time In Service. Only problem here is that one must now start a clock for take off, which is a bad time to divert one's attention to the task at hand. (I seem to forget to do it anyway.) Flight Time goes in his log book (and his pay records). Time In Service is sent to Maintenance to schedule preventative maintenance or required inspections.
There are other considerations beside the engine that require inspections, like the airframe, brakes, etc. These items don't particularly care how fast the engine is turning. There are other considerations as far as engine wear is concerned than engine speed, particularly break-in periods and use of leaded fuel.
You say that "we understand the difference" but you keep reverting to something that does not exist. I defy you to find "tach time" in the regulations. And of all people, a manufacture of glass instruments and solid-state gyros, you should be looking at the future...not what we did in the past.
Your choice to use tach time to determine Time In Service was an error that should be fixed, and it complicated what should be a simple calculation. I'm sorry if I hurt your feelings.
After all, how many years have we been flying airplanes thinking that we are held in the air because of Bernoulli’s Principal. Now we finally have learned that a barn door will produce as much lift as a wing...what holds us up is primarily the deflection of air in a downward force.
To save you some time, here is the definition of Time in Service from the regulations:
Title 14: Aeronautics and Space
PART 1—DEFINITIONS AND ABBREVIATIONS
TIME IN SERVICE, with respect to maintenance time records, means the time from the moment an aircraft leaves the surface of the earth until it touches it at the next point of landing.