It wasn't so much that emp healers were more needed then, but that there were more players who weren't used to the idea of healers not being mandatory.
Exactly this. Almost every mmorpg falls into the trap of both encouraging and also restricting people to the same holy trinity team setup, that complacent gamer syndrome has kicked into full gear. But I think it's also the players that are the problem but more they kind of became the problem.
https://www.youtube.com/watch?v=EitZRLt2G3wIn Balancing for Skill, Extra Credits talks about how "foo strategies" often become go to tactics for lower end players, but they stress a main problem in that advanced tactics have to come somewhat earlier. Many games didn't really or never really do this, and result in players never learning those advanced tactics by the time it's critical to be using them. They get comfortable but complacent, which leads to them trying the same once successful but now losing tactic over and over expecting it to work better and that the game is just throwing a badly balanced spot at them. When of course, in reality, the developers wanted them to move on from that by then.
Healing in Guild Wars, and City of heroes to, was a newbie tactic to keep a team alive. It worked because enemy damage was low but the other defense tactics don't work quite as effective early. But in guild wars, as you got out of Destroyed Ascalon, you were expected to use more skills and have a better variety of classes in your teams. In city of heroes, as you got to level 15ish enemies began using buffs/debuffs more frequently. IMO, both games kind of failed in this, somewhat, for a lot of players. They went into higher levels in city of heroes expecting those aura rockers to keep them alive, but we'd see that either their own character was already good, they got lucky with people using other buffs to carry them, or they got tired of the game or just never picked up it's depth. Or they'd be crushed by non-council, and avoid everything but council under a misconception they were all overpowered and the game was artificially difficult but didn't want to abandon their character.
It wasn't until someone showed them just how effective the non-healer tactics were that people move on. I was lucky to get in a team with a couple of storm and force field users and found that I was invincible and fighting +4's. I was left thinking "Holy shit this is a power house, and theres no healing here!". And I found it wasn't to hard to do when I experimented with a cold domination corruptor. I also found a few others who wouldn't move on from the "foo strategy" and ended up leaving because they never picked up on the games depth. They moved on to CO, then later quit CO when CO turned into a stagnant slog.
I also think though that CoH had an issue with giving people the sense of being more powerful with the buffs early on. Specifically force fields were very discrete, but we all know just how subtle sonic buffs were and how so many people didn't even know they were being buffed. Or even saw the difference in some cases(controllers/corruptors had this pretty bad in the early levels with sonic resonance).
Guild wars 1, likewise, would very severely punish players in the northern shiverpeaks. They wouldn't pick up that monk healing, while decent by itself, was ineffective in the long run. They'd miss the idea behind prot spells. They'd miss the value of mesmer and ranger disruption quite early enough. So when they go to the shiverpeaks, they think "Well I got 6 slots instead of 4...ok one more damage and healer!". Or if they were a wammo continue thinking they were indestructable and slowly get lucky. Even though they were now facing mesmers who'd disrupt the team regularly. It had pvp and pvp tactics evolved quickly, but even then you'd see people quit over IWAY being nerfed very similarly to people in the video quitting over the zerg rush being nerfed. Players didn't have the skill/knowledge to move on to other tactics or learn anything.
