Actually you wouldn't. There are many reasons you would want/need multiple pieces of armour. For one stats, depending on the build you want you need armour with different stats. For instance when I play as my warlock using stormcaller my main build rely's on grenades so I'd need armour that maxes my discipline. When I play as my warlock using Well of Radiance I rely on rifts and my super mostly but my grenade does play a big part so I'd need armour to raise my discipline and intellect while ignoring strength. When I play as my warlock using voidwalker I almost exclusively run Nezarec's Sin so I'd want armour that balances discipline, intellect, and strength. So as you can see I'd need 3 sets of armour to cover my builds. The issue with locking mods to elements is it makes making a build insanely hard since not only do I need armour to match the builds I mentioned but I also need 2-3 versions of each armour piece for each build so I need 6-9 pieces of armour in every slot so I can maintain my stat distribution and have all the elements covered for situations where I need different weapons equipped. I might normally run Arc and Solar armour because I'm running pulse rifle, smg, machine gun. But in a different situation I might need a grenade launcher, sniper, and smg so I need to switch to Solar and Void. With armour 1.0 you could get 1 piece of armour that covers 2 of any weapon combination examples machine guns and grenade launchers or pulse rifles and smgs. So you only need to quickly change a perk or 2 when you change weapons. But now if you change a weapon you need to change your armour piece and potentially also change your mods because it's only possible to combine specific weapons with elements. In armour 1.0 not counting exotics I only had 3-4 pieces of armour per slot depending on if I had my perfect rolls. If I switched a weapon I only needed to switch a mod or if I switched a class I needed to maybe switch my armour piece. Now I have each armour slot filled completely so I can cover all my weapons and class builds and I need to by much more picky of what exotics to carry because I need more slots free for non-exotic armour. There are a few different versions of how we want the restrictions removed. One is remove the restrictions but have it so when you match elements the mod is cheaper or there's a bonus to it. The other is remove the restrictions but have element specific mods. If you look at the artifact mod Oppressive darkness it affects only void grenades so that might only go on void armour or arc battery might only go on arc armour. Restricting all mods to elements just makes making builds tedious and almost forces you to use only 1 build since to maintain so many builds is almost impossible. The odds of managing to get 2-3 sets of armour with the same or almost the same stat distribution but of different elements in order to match your class build and weapons is insanely low. Like you will probably never manage it low.

I suppose if you were a glimmer hound and that cost bothered you. Otherwise you could just switch out mods based on what you were playing. The only difference could be in the base roll itself, but then again I dont envision you master working multiple armor sets, for a few base point adjustments. I believe the intention was to find the armor that suited the class you wanted to play, and maximize. They wanted decisions to mean something, and the drops to mean something. Personally I plan on using 3 characters as builds, not multiple builds in each class. So void warlock with disclipne, and weaken mods, melee hammer titan with melting point focus, ect... not 3 different builds for 1 warlock. But I suppose that is personal preference.

Glimmer hound? Do you know how expensive it is to masterwork even 1 set of armour? It's insanely expensive no matter who you are. You can't really just switch out mods since weapon mods are scattered between the elements. Switching mods out won't have a strong impact since the weapons you're using most likely don't share elements so the only mods you can switch are general mods which are so expensive that any build you make will suffer massively the moment you slot one of them. An issue with stats is they're so low even for base rolls and they only improve at masterwork. So it's much harder to manage a stat build. And depending on the build you want you need specific stat distribution. "I believe the intention was to find the armor that suited the class you wanted to play, and maximize" the issue here with that is most people like using multiple classes. And depending on what you're doing you almost need to use specific classes. In a raid it's almost required to have a Well of Radiance or now Ward of Dawn. But not many people want to exclusively use Well of Radiance all the time to make it worth only having armour suited for that build. The same goes for not everyone wants to run bottom tree gunslinger with nighthawk. Depending on what you're doing you need to use different classes otherwise you're going to have a bad time. If you're doing something that's a long hard fight Well or Ward is basically a requirement, if you're doing something that needs you to do a ton of damage quickly Voidwalker or Nighthawk Gunslinger is basically a requirement, if you're doing something where you're dealing with tons of enemies a lot Stormcaller, Nightstalker, or Shards Bladebarrage etc are basically a requirement. Making it so they want you to pick one class, stick to that class, and build for that class even though you have the ability to freely use any subclass you want and using different subclasses is needed is kind of a dumb idea.