If you have access to a long range, I'll play devil's advocate and say that I think many people build them just as an excuse to spend $5k on another toy and rationalize it as a "trainer". That $5k is almost 4000 rounds of 6.5, not to mention to get max accuracy from your .22 it's going to be $15+ per 50 for match ammo.. If you only have access to a 100-200 yard range, can't get to a longer range etc. and you're on a budget, building something under $1000 then maybe there's some benefit. However, unless you have the same chassis, optic, weight/balance, trigger, and bolt throw it's not an ideal trainer, and that means it's not going to be cheap. If there's one thing talking to olympic level coaches for shooting sports has hammered home it's that consistency is key, and you don't get that shooting something that's not your tournament gun.
When was the last time you saw someone say "I'm gonna build a .223 or .308 as a ".338 trainer" because at 400-800 yards it's like a 338 at xxxx-xxxx and it's cheap ammo"? I'd bet never.
Don't get me wrong, I love shooting accurate .22's they are amazing fun, lots of great .22 matches are popping up, and many of us can't easily get to 1000+ yard ranges, so there absolutely can be benefits to them. They are great for getting kids going, great for new shooters to "break in" to noise/recoil, but I just don't buy that for most of these rigs I see spending several thousand $'s on "trainers" is really about "economy" and training.