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.