That's fair enough Han, I have nothing intelligent to add or disagree with your post and will get to the Liu bei discussion today (bar unforseen events)
I'll even uses men like Zhang Kai, Wang Zhong or even Xiahou Mao as my vanguard and have them weaken the enemy forces before sending my strong and loyal general to finish the enemy, well as long as they get merit and reward from the effort, i doubt they going to complain
In fairness, Zhang Kai may have been a skilled assassin, Wang Zhong was probably given posts (bar his local posts before the war) for bringing a thousand followers to Cao Cao and was something of a survivor given what he had to go through. Xiahou Mao... yeah best I can give him is friendship (which a ruler needs), family ties (useful for a ruler) and there wasn't complaints about his civil work (then again, good luck to the complainer on that one). So such men could serve a purpose even if miliatry affairs ill-suited both or it was a one shot achievement that they lived off on ever since. So your basic idea of trying to use all men isn't an issue, warlords had some horrible men tolerated under them due to their talents and importance to the army
Using them as vanguard would have been a bad idea for 3kingdom warfare, particularly in the early stages. Vanguard loses and the army risks complete collapse even for experienced armies like Gongsun Zan's. You need your great fighters at the front in that era
Hence the word, my top list of hated person is a person who are useless or treacherous. Shortlived are considered useless to some degree but i'll make an exception of that due to the circumstances. Treacherous is slightly less as long as they can be reason with to help me. Even Lu Bu himself still can be reason with even though i would wary of his murderous streak after he shown the intention in killing me.
I'm careful with useless personally, sometimes they get one shot in spotlight and it goes wrong and talent may have been elsewhere, some simply lack the records so one doesn't see the success if they served outside the 3 kingdoms themselves. Some people are suited to certain things (philosophy, art, scholarship politics) but not other things (miliatry skills, actual administration) that directly suits ruler, or have a crippling personality flaw.
Mercenary generals like Lu Bu, even when they were distrusted, could be of considerable use for a spell. It is a calculated gamble of the cost and risk it inevitably brings vs the benefits. I suspect a "depends on situation" is best with those.