It's time to pick heroes for the mission at hand, and you need to pick three. The choice of heroes is, of course, dictated by what you want those heroes to contribute. So what do you need from heroes? Damage? Everyone and his neighbor's dog can deal damage. So can every single henchman in the game. Healing? Plenty of henchmen can do that, too. What's the point in bringing a hero if you're going to use the hero like a henchman?
No, if you're going to bring heroes, you might as well bring heroes to do the things that henchmen can't. Enchantment removal? Well, a necromancer or a mesmer can do that. Interrupts? Try a ranger or a mesmer. Armor-ignoring damage? How about an assassin or a mesmer. Hex removal? Bring a monk or a mesmer. Health degeneration? Necromancers and mesmers have quite a bit. Slowing hexes? That would take either an elementalist or a mesmer. Energy denial? That's nearly the exclusive domain of mesmers.
See a pattern here? Whatever you need that henchmen can't do, a mesmer hero probably can. You could bring a monk to remove hexes, an elementalist to snare mobs with hexes, a ranger to interrupt mobs, and a necromancer to remove enchantments. Or wait, you can't, because you only get three heroes at a time. But you could bring just one mesmer hero to do all of that, and have two hero slots left over.
So what about damage? People who don't know how to play love builds that do lots of damage, right? Mesmers can do that, too. See, most mesmer damage is armor-ignoring. While other heroes may hit heavily armored mobs for 5 damage per shot, mesmer damage skills do the full rated damage. In other words, they're most powerful precisely when you need something powerful. After all, do you really want to breeze through the easy areas only to wipe on the hard ones? Whatever makes the hard area hard, a mesmer hero can probably counter it and get you through.
What if mobs are making your life miserable by what they cast? Whether it's heavy damage spells, potent heals, or whatever, the solution is simple: don't let them cast it. I'm talking about interrupts, of course. And this is where you really need heroes. It's a simple fact that AI has faster reflexes than humans.
There are, of course, a lot of classes with interrupts. Warriors can interrupt a lot, if they happen to be in melee range. Rangers can interrupt quite a bit, too, provided that the shot doesn't get dodged. They both need to actually hit, as opposed to missing due to blindness or getting their interrupting attack blocked. Some other classes can interrupt, too, perhaps when their next martial weapon attack finally comes, or at the end of casting a spell, by which time the enemy cast you wanted to interrupt is complete.
Mesmer interrupts don't have these problems, of course. Mesmers get the full spell casting range, ignore line of sight, and get the cast off in under 1/4 second. Depending on your fast casting, it will typically be more like 1/6 of a second, or fast enough to interrupt the enemy cast before it completes, rather than after.
And mesmers also have a huge variety of interrupts, allowing you to bring as many as you need and of whatever types you need. There are spell/chant interrupts, attack interrupts, skill interrupts, action interrupts, or even a way to interrupt all mobs in the area.
Some will complain that heroes waste interrupts. But interrupting isn't the only purpose of mesmer interrupt skills. On top of the interrupt, you can add damage, energy management, energy denial, or disable skills, allowing you to greatly harass mobs even by interrupting spells that don't particularly matter. Is it really such a waste to interrupt fire attunement if the interrupt skill also completely disables all of a potent elementalist mob's skills for 14 seconds?
So why do some people still think mesmers are so useless? The answer is quite simple: they don't know how to play. The effects of mesmer skills tend to be more subtle than those of other classes. A minion master has quite visible effects, for example, as you can see the minions. You can see a ranger fire a bow, a warrior swing a sword, an elementalist shoot fireballs, and the like. But unskilled players may not notice mobs not casting because they're interrupted, or missing because they're blind, or having their health drained away, or hurting themselves more than your party with every attack.
Some people will not notice all of the good things that a mesmer hero can do for your group because they don't know what to look for. Instead, they prefer the weaker skills whose effects are clearly visible over the stronger skills whose effects they do not see.
This is why they're stuck on whatever mission is at hand and begging for a run through. There is no need to be like them. A mesmer hero can do many good things for you, even if you don't notice them. Consider bringing a mesmer hero on your next mission, outfitted with skills to counter whatever the mission throws at you.
See you on the battlefield,