Most of the backlash against it comes from one of two things:
-Backlash against the kind of people who normally bring it up, who usually deserve a
at best. These are usually the people who, when they first decide they want to try armoured combat, make up "unbeatable" strategies where if their opponent tries move A, they'll just do B, C, and D, and then win EVERY FIGHT!
-Backlash against anything done in the SCA, usually learned from John Clements.
While there have been a few notable practitioners of dual-wielding in the SCA, especially here in An Tir, and there are numerous experienced fighters who have used it at some point in their fighting careers, most choose sword & board in tourney fighting as it is simply the easiest and most effective style (when knees & shins aren't legal targets- just to forestall Mer's complaint). Here are the steps in learning how to effectively dual-wield:
1. Learn to fight with one weapon.
2. Get good at it.
3. Learn to fight with one weapon in your bad hand. This is about as hard as learning to pitch with your other hand, or learning caligraphy with your other hand.
4. Get good at it.
5. Learn to fight with two weapons. You actually have to learn a completely different style of combat here, and using one-handed techniques with two weapons simultaneously won't cut it.
Not only is there 3 extra steps AFTER you've learnt how to fight sword & board, but those are the three HARDEST steps of the progression. When fighting one-handed is a more effective style, you're basically tripling the amount of work you need to do so that you can be a WORSE fighter. The only reason so many people learn to do it in the SCA is because learning how to fight in a style teaches you how to fight AGAINST that style. This is also why we recommend that EVERYONE learns sword & board first, since a large majority of opponents you face will be fighting in that style.
Of course, this is "dual-wielding" in the true "two offensive weapons" sense. Using an axe or pick held just below the head, an inverted spear, or an inverted longsword held by the ricasso as a defensive parrying tool with serious offensive possibilities is much more common, and arguable more useful. If missiles aren't involved, I'd prefer a longsword in my off-hand to a shield, except that I've spent much more time with a shield, specifically a heater, so I'm currently more skilled with it.
By the way, here's a most excellent paragraph
I found when I looked up "Florentine" in Google:
Florentine was first used as a term for a weapon style within the Society for Creative Anachronism circa A.S.2 (1970 AD) to describe a fighting style involving the use of two pounds of spinach and a pair of salad forks. Later the spinach was either discarded or eaten (feasts often started late in those days) and the term came to denote any two-weapon style, or, alternatively "what medieval knights would have called fighting in tournaments with two weapons at once if they had ever done such a thing, which they didn't". The style is sometimes referred to as “Too many swords.”