I'm getting quite fed up with the AI excuse.
In Infinity Engine games, friendly fire was always on, and it worked perfectly fine.
Somehow enemies didn't kill each other mindlessly, system was completely symmetric (as all D&D based games) and the game was challenging but fair.
Are you telling me, that developers cannot do the same thing now, when average CPU has ~50 times the power it had then?
Somehow creating a symmetric and fair combat system with friendly fire enabled and balanced for both the party and enemies is impossible now, but was possible 15 years ago?
Don't say to me, that 'humans are smarter than AI', because we have chess engines that consistently win or draw against everyone, including the best Grandmasters.
I'm pretty sure we can program AI to make decisions at least as good as an average player.
Enemies in BG2 most definitely did not play fair. Between admitrary immunities for human enemies, doing 20-30 damage with 1-8 weapons, and Mages storing like 4 protection spells in one Contingency, the AI did not play by the player's rules. It was less dramatic than in modern games, but the playing field wasn't fair. Don't get me started on the arbitrary BS the bosses in Throne of Bhaal got. And, It also resulted in a gameplay that was mostly about destroying your enemie's protection spells then killing them fast with physical damage classes. It's a way to play, but Bioware is obviously not that fond of it now, nor are even old school focused studios like Larian because they didn't do it either.
The mage AI was also quite stupid and would happily lob fireballs in a crowd where the player characters had fire resistance.
About chess, that's not an argument. First, the computers that can beat grandmasters are million-dollar supermachines purpose-built for the task, not your average PC. Second, chess is a game that has a very, very limited number of variables and possible commands compared to even a bare bones video game, let alone something as complex as an RPG that has thousands (at least) of possibles actions in any given situation. AIs actually aren't AIs, they are a bunch of scripts that sometimes draw information in databases. If true AIs exist, well, they won't put them in video games first that's for damn sure.