I feel offclassing is under utilized, it's mostly seen on last defenses, sentries/heavies popping up, sniper trying to hold angles, very rarely you see an offclass pushing last that isn't a spy or sniper when a team gets desperate or bored of suiciding in and stalemating. An engie pushing with a lvl 3 is considered toxic, but it allows the other players on the team to keep moving forward and have some backup on a failed push.
But also unlocks used on the 'regular' classes are too ingrained, you rarely see kritz ran, every demoman uses iron bomber these days, barely see any quickie-bomb launchers clearing stickies from points or even soldiers running banners.
The refusal to allow any unlocks which could speed up the game even more (disciplinary action, quick-fix) that could make trying to run other classes viable is imo dumb. The sheer fear of a possible heavy to mid when a pyro with the current whitelist can already show up with backburner and thermal thruster + powerjack at the same time...
It is probably the fact that if one team starts running offclasses you are also forced to rock-paper-scissors to counter their offclass; and if your players are uncomfortable playing anything but all aim-no-brain scout, mge-only soldier and keep dying to the offclass they start tilting which doesn't improve comms to possibly hold or counter the offclass. Encounter a heavy on mid? Try to come up with a strat how to counter that, improve comms because of the enemy trying dumb shit you now know how to stop..
TLDR: Refusing to only play meta 2-2-1-1 is a legacy mindset; certain stalemates can be broken more easily with an offclass and/or unlock that are currently not used enough to surprise the enemy team. Refusing to scrim a team actually trying new things = dumb, that is what these practice games with 0 consequences should be for!