For the stage (I'm talking from a stage perspective, not a PvP perspective), being Eir requires you to heal other people. Eir is probably the hardest character to play as in the game in terms of your role in the party. You have to constantly keep an eye out on what EVERYONE, both your party monsters AND the monsters, are doing AT ALL TIMES. When you are Eir, you are FORCED to move in and heal people when they need it, unless you want them to die. ANY other class can CHOOSE when to unleash an offensive or not, but Eir is the only character in the game that is FORCED to enter into the situation as it arises. An Eir that's in the middle of combat (which she should be to contribute maximum amount to the party) needs to be ready to break off and heal at a moment's notice, and sometimes this can happen at VERY inopportune moments. Being forced to heal someone (or else they'll die) also forces you to move to a certain position, which can be restricting and even get you killed a lot of the time (I'm sure many healers have lost count of the times they rushed in to heal someone only to get SLAMMED by a monster as soon as they did and killed). Healing spells are also the most mana-expensive skills in the game (so potions and mana recovery can't keep up much at all), and a stage Eir needs to sacrifice her own HP often in order to keep up with the mana output in general, whether she's FS, hybrid, or battle. Combine this with the character who has the lowest durability in the game (both in terms of HP and defense, not that the latter is that particularly significant, though it does make a little difference) and terrible defensive skills in the game compared to other characters (she doesn't even have ANYTHING that gives her invincibility frames besides Light Shield that you get at level 54 and has a 3 minute cooldown and the highest reagent cost in the game of any skill) and you have a character that requires a lot of concentration and playing skill to fulfill your role in the party. Her melee range one of the worse in the game (only Tia's is shorter, but Tia hits a heck of a lot faster) and so is her melee damage.
Worse defense, worse HP, worse melee damage, 2nd worse range, highest mana cost on her skills.... really, it's as if the developers knew that parties would NEED healers, so they purposefully went out of their way to make healers the worse at everything to make up for that. Actually, given that they probably also purposefully went out of their way to give potions obscenely long cooldowns to make any venture without a healer an exercise in frustration means that is probably the case.
When I started playing other classes besides healers, it felt like half of my brain simply had nothing to do anymore. Not having to give a crap about how much health the other party members have and being able to focus and concentrate 100% on just killing stuff (or maneuvering stuff in ways that other party members can kill them) does that, I think.
Currently I'm main-ing two full-time fisher-girls. They fish. A lot.