I did a couple of thousand hours flying the J31 TP before moving onto the B757 in 1994 .. to me it was very useful experience and formed part of a recognised career progression, then.
I think it is a great loss to the industry that this stepping stone seems to be lost, as it all comes under the heading of airborne experience .. which cannot be taught in the classroom. Flying TP's sometimes places different demands on the pilot, but any differences are easily overcome in a mature training environment .. and a good TP Captain/FO would make a good Jet Captain/FO in the fullness of time.
Having worked for many companies over the last 32 years, it is the variety of experience that makes up a balanced airline. No one background should be exclusive, and no other ignored ... Ex Military, cadet and TP all have their place, as they all bring something different to the table .. and that diverse background of experience should be treated as an asset. Having flown with all of the above extensively I would have no preference for any group ... more their professional attitude and relaxed demeanour individually.
If I were to start again, my Military experience would be less in demand as would my TP time .. I would also need much deeper pockets. From the smallest to largest aircraft, I have enjoyed and respected the bonding system, never having paid for a type rating ... this sadly does not seem part of the general model anymore, and I think that a shame. I know I have been lucky with timings and opportunities ...
In precis, there are good and bad from all backgrounds .. it is the variety that is needed ..