It doesn't really matter what you think - valuations are put in place to protect the asset of the insured ie their classic car in the event of a insurance claim. Agreed values are just that - a value of the car agreed by the insurance company in the event of a total loss or requiring serious accident repairs to put the back to pre accident condition - not improve it, but pre accident condition.
Market value is just that- the current value of the car at any particular time relative to the market.
If an insurance is happy to pay out the agreed sum, or the market value following a valuation by a club official, what is the problem you have with that? Without a valuation, the insured has an uphill battle to fight to get anything like the current worth of their car. So for the £20 fee charged, its really a no brainer...
And as for vehicles from the last 30 years or so being increasingly difficult to repair by 'relatively unskilled' owners, hasn't that been the case for longer than 30 years as cars have developed, and become more reliant on electronics controlling every aspect of their operation.
But going back to the 2.5PI, back in the 70's, not even Bristish Leyland / Triumph garages fully understood the mechanical fuel injection system, and how it operated, often being unable to correctly set them up, even with the proper equipment. But fast forward a few decades, there are plenty of specialists who have overcome the issues, and now there are plenty of smooth running examples (including TR6s), where the owners haven't been put off from ownership purely due to the more intense maintenance required. In fact they are more popular now than they ever have been.
I think your view is rather negative. If everyone who sought to run an older vehicle with 'so called' modern electronics controlling it, or an older classic with difficult or ridiculously expensive spares / parts (such as Fords), then no one would ever consider owning such a vehicle ever again.