Interesting stuff.
Oh man, this is a separate debate, but come on. 31-count kata can be done in 15 seconds even with a partner, and most western martial arts recreations I've seen also run at about two strikes per second once an exchange gets going, with exchanges of 3-4 strikes not that uncommon. And that's what you pick up in a year of training, which in D&D is basically 1st or maybe 2nd-level Fighter.
D&D has never, ever been even remotely realistic about time spans of combat.
The justification of the multplie/Bonus/extra attack in the mechanics was explained once somewhere (1E/Dragon) as 'you swing X amount of times in 6 seconds and of those you have Y chances to land a damaging blow'.
Pretty sure one of the reasons Feats are an optional rule is that they disowned "balancing" them with or against anything; like multiclassing, just not on the R&D radar, use at your own risk.
Officially they are balanced and are optional to allow the pre-3e fans to be happy. Multiclassing (and pre-4e fans) are why characters are so weak at levels 1-3 so multiclassing is also theoretically balanced.
I forget where it is in the books, but there's an explicit entry somewhere that says you never stack the same bonus.
A friend of mine pointed out recently that in some respects, 2nd Edition AD&D was the best-balanced version of the game so far - because the focus of balance wasn't between PCs, but between characters in the world. The limitations inherent in the system (level limits, class restrictions, etc) were built around world-building and the relative power of societies.Yep. I mean, D&D wasn't close to having every single thing balanced for literally decades, and millions of people had fun playing it. I'm pretty sure they probably thought, "Way more people will have fun using these X, Y, and Z things we're putting into the game than people who are 'Errrmagerrrbb everything has to be balanced to the 0.001 DPR!', so rather than not do it at all, let's include them. After all, rules are guidelines, and the great thing about D&D since day 1 is that every table can use what they want to make their experience fun. "