Except you are now slotting 2 damage-only abilities whereas 1, if you invest fully into that resource, could do 6000 dps itself--and if it did less, say 5000 dps, your open ability slot could easily fill that missing 1000 dps and then some. You'd effectively need twice the ability slots or a full-spec damage output for Stamina AND Magicka. Neither are possible.If I use the best Stamina and Magicka abilities, and split my resources, I'll be doing the most damage. Stamina ability X does 3000 damage per second. Magicka ability Y does 3000 damage per second. I'll be doing 6000 dps.
By splitting your resource replenishment, you're compromising additional utility/damage in favor of sustain. If you only had Magicka to worry about, sustain would be a secondary thought. You'd consider more damage because your potions and heavy attacks would keep you covered. You'll heavy attack when it's IDEAL not just when it's NEEDED.I can keep up both my Magicka and Stamina resources no problem. Invest in a little recovery, or Grundwulf, etc. I'll heavy attack every now and then with both weapons too.
What is more likely: A developer implements a few hybrid pieces to see what the players do with it, but ultimately can't invest in a 3rd playstyle for the player base due to the complexity. OR. A developer implements a few hybrid pieces in a multi-year plan to make their viability realized, waiting for enough players to commit. The former is what happened. "Hybrid" sets are good in specific scenarios, mostly solo content (even Maelstrom Arena), which completely caters to the lone or fantasy player playstyle. To realize that hybrid sets are for solo content means that non-hybrid sets are for multiplayer content. That seems fair and balanced to me. Solo players are content being able to complete something alone, even if it takes a bit of time to do it. Multiplayer players are content being able to complete something together, even if it takes optimization to do it.Hybrid sets exist in the game, therefore hybrids are being worked on to be a thing.