I don’t really get your point cargo. Ok you don’t need a medical to get an ACU licence and if you let it lapse for a couple of years you can re-apply at the same level.
I have looked through the MCUI form and at a quick glance I didn’t see anywhere where you have to record your previous years racing results. In which case you could just keep reapplying for your licence do your medical each year and have not raced for 5, 10, 15 or even more years!

So really the only difference between the licences is the medical.

My personal opinion is that the TT licence had nothing to do with the TT or TT riders but was aimed at MGP competitors. It does prove that you have been racing in the last year which neither ACU or MCUI licence does.

Does it make a rider more prepared to face the challenges of racing on the mountain course, no!
Does the fact you’ve passed a medical make you more prepared to race the mountain course? On the basis of what I’ve seen in all aspects of my life over the last few years, no.