Is it poor form for reasonably intelligent enemies to focus their fire on the most threatening yet least well-defended...

It's poor form if the system you're using arbitrarily disallows the badass with the shield to protect his caster/assassin buddy, ala Pathfinder or some DnD. This is because the system already actively inhibits cohesion and tactics on the players behalf: otherwise, go wild, because a wizard shouldn't look like a god damn wizard if he wants to be a wizard for long.

That being said, deception checks have their own problems, as sometimes a character can be two things at once (what, are you really going to have the Scion to the Duchy of Bumbuck, who happens to be a wizard, have a deception check made against him if he's in noble clothing?).

My issue with this is that it's not always clear who is the support and who is the infantry, or that even intelligent foes are familiar with this kind of conflict. IMO small party combat involving caster/assasin types is fairly rare in most settings, too rare and unpredictable for 'geek the mage' to become common knowledge.

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The idea of kill the most dangerous person first has existed for who knows how long. It's just most noticeable now because weapons are at their most dangerous now and chances are, the support is the most dangerous. (In dnd, casters and shit) and hell, it's even harder to tell in modern warfare who's who because the only difference in how people look is what gun they're holding and that's kinda hard to tell aside from they are holding some kind of rifle.

Same guy here
Though obviously someone who hasn't had much experience fighting won't know this

Also to me it just seems 'wrong' for mages and bards (less so rogues) to have to hide their class trappings to be tactically optimal. Sure, it kind of makes sense for the wizard to cast Disguise Self so he looks like another plate wearing fighter if the party is ambushed, but what about his wizardly pride? In offical art, we don't see much sign of this behavior. At mid levels, does everyone fight in disguise all the time? While we're at it, seems like it might be better to implement an Aggro system than have to constantly second guess NPC motivations for dps targetting. Or... just have smart enemies default to one of a few reasonable target-agnostic strategies, then let them adapt as seems appropriate.

There's nothing wrong with playing to genre conventions, IMO, instead of some abstract ideal of 'realism'.

Smart foes know to geek the mage, but only a truly skilled warrior can get around the fighter to engage the wizard in melee. Assassins prefer the softest target.

It's generally poor form, because it's hard to do it in a way that makes sense for the enemies. As points out, your enemies' primary goal should be to survive, unless they're absolute fanatics. Running past the armored fighter and getting a sword in the back, then ignoring her hacking at them while they in turn hack at the mage, only makes sense from the perspective of pawns being controlled by an overmind. From an individual perspective, it's pretty much suicide.

Which isn't to say that suicide isn't sometimes appropriate. See stories of soldiers charging machine gun nests and getting gunned down while they take it out. But that should be the exception, not the rule.

I'd limit such tactics to if the enemy had an officer or some other organizing force that is giving them tactical orders, otherwise they devolve into "hit man closest to you"

Like knowing that a troll is weak to fire, it depends on what knowledge they have in-game. I mean, a good assassin or rogue would look like an unassuming commoner most of the time, as do most bards and monks. If the character is obviously dressed like a wizard or sorceress they're fair game, but like the PCs shouldn't know the statblocks of most "out there" encounters without meta-ing, neither should your encounters. But I must stress that I do like it you're treating your encounters like an opposing team of PCs, even if their stats don't match up.

Of course, this goes both ways: if one guy looks like a low-level peasant but he's in a party of beefy armoured dudes, he's obviously more than he appears.

First rule of thumb: it should be obvious to any intelligent creature within the first round or two who the most dangerous person in the enemy group is unless they're intentionally holding back.

Second rule of thumb: if the enemy group has a caster of any kind, the whole group knows that casters are the powerhouse.

Third rule of thumb: all but the most highly trained soldier will ignore the first two rules of thumb in the heat of combat if there's a screaming half-orc shoving a waraxe in their sternum.

Depends on how "hard" the encounter is supposed to be.
If it is a softening, early mook fight, I tend to rule that they are dummies who go fight the big, fighting man, since the skinny man in the bathrobe couldn't possibly be worth worrying about, or else it is just a matter of who is closest and yummiest.
For "harder" encounters, where there is generally some form of leadership, medics and support casters are generally their priority 1 unless otherwise noted. The exact same encounter can be a cakewalk or a terrifying ordeal depending on enemy intelligence.

Not much at low levels, but higher level ones should know better.
There is no problem with Illithids looking for casters and having protections against assasinations, it's part of their lives.

Mind you, it does run into the fact that generally wizards tend to be a non-hardened source of serious offence and support.

So while it is risky to turn your back on the sword swinger it's quite justifiably less risky than not getting up in the cloth casters face.

This is part of why clerics are healers AND tanks.

An averagely setup encounter does not have the status ailments to quickly lockdown a beefy healer, but a smarter party might.

>medics
That's against the Geneva Convention.