I even had a level 19 Tristan and I think he participated in like 5 missions throughout the run lol. In general this game is absolutely fantastic, but one of my gripes with it is that after the early game, you basically build around 6 heroes and never use the remaining 6. The reason is that, even on Very Hard, you generally don't take many injuries or much vitality damage, and when you do, your Cathedral and Hospice are so good it'll only take you a mission's time to get heroes up and running again.
For the last third of the game, I never used anyone but Mordred, Galahad (or as I like to call him, King OP of the Unbalanced), Morgawse, Ector, Balin, Tegryn, Black Knight and once in a blue moon White Knight.
I wish it was harder to avoid Vitality damage and injuries late, and that they took longer to cure. Generally it's a problem from midgame forward that fights snowball very hard - if you start taking Vitality damage you probably lost the entire fight - conversely, fights that you win you often win flawlessly (i.e. taking very little damage). They recently rebalanced late game enemies to deal less damage and have more HP which helped this (at least for me, some Formorian missions actually forced me to lose Vitality and I could still win because enemies didn't just one-shot my guys), but it's still not enough to make you use more of your bench.
EDIT: It's also hilarious that they uber-nerfed Vanguards since I last played, yet I didn't notice it until I read the patch notes by coincidence, because they are still easily the best class :D. As long as I have a strong Vanguard on the team the other three Knights can be a mixture of whatever. You don't need a Defender by late game (in fact it's probably more optimal not to run Defenders, but I like playing with 4 different knight types).