Yeah, we came to the same conclusion that, at fairly low levels (say 5+) that -2/-4 didn't actually matter that much. By that point, you probably had magic weapons, and an 18/percentile strength fighter was hardly rare. Considering that most of the things you fought at those levels were around AC:5 (ish), taking the to hit penalty was worth practically doubling your damage.
By 2e, 2 weapon fighting became the default in every group I saw. It was just head and shoulders better than any other option.
Well, yes and no, I mean, -2 is -2 is -2, lol. You have to do the math and see, for a given scenario if your DPR goes up or down when you accept that penalty. Only the "you get to cancel this out with the R/AT for your DEX" really just wipes it out (since you would not get that as a bonus anyway). So, if you have a 17+ DEX then (AC issues aside) you are 100% better in any version of the rules to grab an off-hand weapon. Worst case you miss with it! Even if your STR is 18/00 without that DEX adjustment, you're still hitting less, and those are hard hits. OTOH extra hits are hard hits too, but at -4 they better be REALLY hard. I'm not going to try to dredge up all the math, it is straightforward anyway. Nor does multiple attacks per round change anything, if you are getting 3/2 attack routines, great, each one stands on its own for DPR purposes.
Even with UA weapon spec rules all of this holds. It is only the contrast between an attack with a single weapon and the two attacks with the double weapons that matters. If the damage per attack routine is more, then dual wield, else don't. No specific STR level is going to change that, and you have to make some assumptions about the AC of your targets too. So it may be less desirable to dual wield against the toughest opponents (also if your weapon can inflict special effects on a hit, that might also discourage dual wielding). Obviously you get more bonuses if you double specialize though, so there is a lot more chance you will hit stupid often no matter what, so it was definitely more favored.
2e's version just has the problem that a high level fighter is paying the penalty every attack routine, but only getting the benefit in one of them. Again, high DEX pretty much makes it pain free, and the fact that DEX bonus races are less capped and easier to qualify for doesn't hurt. OTOH the armor rule for F/MU does put a monkey in the elf's wrench so to speak. Elven chain is nice enough, but AC0 full plate is still a lot better! lol. Stack that on top of dropping the shield and it does hurt.
Still, DEX builds are a good bit better in 2e overall. Magic items are also significantly less common if you play strictly by the 2e rules, though since most people were running modules and whatnot designed for 1e that didn't ever seem to really happen.