I wonder.
I can see the point that a good task for a Mesmer in certain situations is to cast off target in order to disrupt some irritating spellcasters in the background. I do this a lot.
And I can see the point that there is no point in a Mesmer hexing a foe that is about to die anyway because 3 Warriors and 2 Rangers are beating on him.
But I have played a lot in PVE parties where there was only one Warrior, or even if there were more, it seemed to take an effort to knock down each foe. And in this case, and assuming there is no more irritating spellcaster around, isn't it better to aid the foe that the others are trying to kill quickly with a little bit of -HP degradation or maybe a little bit of attack blocking? Does that not help in this situation? Or doesn't it matter in the least?
I think one thing to remember is that this is a game, at least the PVE portion of it, and Anet has obviously created the Mesmer profession to make the game more interesting. It could be argued that you can do without a Mesmer at any time, taking instead an extra Warrior, or an Elementalist, or whatever. But people play Mesmers, and it adds diversity to the game. And diversity to the team strategy, if the team cares to work together as a team. I might say no to having too many Mesmers on the team (actually, had a lot of fun the other night with 3 Mesmers in a party of 6) but I would never say no to at least one Mesmer.
I play a lot with only henchies with my Mesmer, simply because I can do things like Elite skill hunting in a relaxed pace. Maybe the henchies at level 20 are darn good, but for some reason, I seem to do extremely well with them when I equip the correct skills. Maybe next time I go out and do this, I will target, but not cast any spells, and see how well the henchies do without my Mesmer help
What is confusing about echoed chaos storm, by the way? It may not be the best thing to do, but when I go out with only henchies in an area filled with large mobs, it seems pretty effective when I cast two times chaos storm near my tanking warriors. I think I am giving 12 damage times 2 per second for a lot of seconds = a lot of damage. Makes it easy for the warriors to clean up after. Nice thing here is that the computer enemy doesn't know enough to move out of the chaos storm area. And yes, doing this with an elementalist would maybe be much more effective, but this is me playing my Mesmer, not me playing my Elementalist, and besides arcane echo and chaos storm, I have 6 other more diverse skills equipped.
Regarding Rangers (previous post): I am playing a ranger now together with some friends, and am just about to ascend. And I would say that rangers are great! I am equipping a trap, and distracting shot and savage shot among others, and I can really disrupt a lot of skills and spells with the latter two. If I can get away with it, I set a trap at the start of each battle and even during the longer battles if possible, and it has proven extremely helpful in case the monks and elementalist get mobbed (I tell them to run past me, since I stand where I set the trap).