Dual spec isn’t just convenience. It is a large change to how the game plays.
Me as a healer for example here in Classic. I use mana runes on hard raid content when major mana potions alone aren’t enough. I have a hard time farming them in my resto spec. What I did back in earlier phases was grouped up with a DPS friend of mine and go kill satyrs together. One time it was a warrior and with my WF totem and healing him we had basically zero downtime. I kept the runes, we split the felcloth and herbs in the area and both made a good bit of money. We did the same thing in DME together. Now in phase 6 felcoth is worthless but we’ve done the same stuff in Scholo. I get a tank and some DPS that I raid with, we’re all naxx geared so we go and crush Scholo. The tank and DPS get bone fragments and crypt fiend parts for their leg enchants and I get the runes. This is quicker for our DPS if all the non instanced farms for bone fragments or crypt fiend parts are camped and quicker for me than grinding satyrs in resto spec.
I did similar stuff back in TBC when I was a healer. Grouped up with a prot pally friend while he AoE grinded the mobs outside black temple. With my healing and Wrath of Air totem he killed stuff faster and with little downtime and we split all the loot. Later on since he was a raid geared prot pally people would pay him for Heroic Shattered Halls runs. I’d go with him and as fast as we blew through that place the raw gold that dropped was profitable.
This was part of the game. If you were a tank or a healer you relied on your friends for help. That is a good thing. it is an MMO. You could always pay back the favor and tank or heal a dungeon for them. Dual spec removes this as a social aspect of the game.
I didn’t play on PvP servers but from my friends that did being caught in a PvP fight while in your PvE spec is part of the challenge. That is part of what makes world PvP different from instanced PvP. With or without dual spec people will respec for instanced PvP, but out in the world you catch them in their PvE spec. People have to improvise and adapt and try their best to fight you off while in a suboptimal spec for it. Here in Classic I’m not on a PvP server but if I was I would have to go out herbing in my resto spec. Even though resto isn’t the ideal PvP spec I would have to learn how to attempt to fight in it in case the herbs I wanted were contested. With dual spec I would just switch to my Ele/resto PvP spec everytime I went out farming. That changes how world PvP works.
This isn’t even touching player identity. Respeccing being rare meant your spec formed part of your identity too. You weren’t just a paladin, you were a prot paladin, the best one on the server, the one everybody wanted to tank their heroic shattered halls run.
Dual spec isn’t just a convenience feature, it changes social elements of gameplay and player identity.
All this being said, I could get behind something like a “PvP spec”. You go to your trainer, set your “PvP spec.” Any time you zone into a BG or an Arena you are automatically switched to your PvP spec and then switched out as soon as you leave. Respeccing for PvE remaining unchanged. This doesn’t cause any kind of gameplay change, as people almost always respec for instanced PvP anyways.