I'm too much of an L2 simp to want healers/supports doing any kind of dps. You balance the game around party play? Well then make it so that dps can't survive w/o supports and supports can't do shit w/o dps. Though obviously that kind of design would probably not fly in this day and age.
It's not a good design though.
There are two paths. Tell me which sounds better to you.
1. Tank and Healer/Support together are about equal to one strong DPS class but only get half the exp if they manage the kills, slow and steady.
2. Tank and Healers/Supports are even together just 20-30% of a single strong DPS, if that.
In case #1, more people are incentivized to play Tank and Healer and become friends with a bond, and work on optimizing their skills in all areas, DPS, Healing, Mitigation. Then, a good DPS can join them, and everything flows relatively smoothly, expanding their group. Add more DPS and soon you have good reason to seek out stronger enemies.
In case #2, in order to be efficient one must make a larger group, seek DPS first, or 'reach the point where the Tank is not dead weight compared to just throwing more healing Mana at a strong DPS' (healbot). The relationships still form, but the core aspects of it aren't as cohesive and if the game's design is good, you still can't necessarily just 'add more DPS and go up the ladder' of enemy difficulty because the Healing Mana consumption rises too fast.
Healers and supports need to be capable of decent DPS, and it is safe to design them that way. Does anyone ever really think "Man let's get a party of just healers together since we do decent DPS we don't need any of those hard-hitters"?
There's 'social incentives' but you have to be careful about which ones you trigger. Making everything a situation where Tanks or Healers 'have to rely on DPS' or vice versa leads to situations a lot more like GroupFinder outcomes. It wasn't that way years ago, but it becomes that way quickly now, and the two social incentives are somewhat opposed.
You don't want people to 'have to form a pickup group at near max size to be functional', because it forces impatient people to group with easygoing people and can cause internal friction types that lead to declines in sociability for both types.