It's different in a turn-based RPG. When you decide to make a different class heal another, you severely have to take a hit in damage since you sacrificed a whole round of that class doing their job to do the heal. Made even worse since you can only do it to 1 single party member. And healing items where you can heal as good as a true healer were not common at all. True healers had the exclusive ability to do party-wide heals, in those games, as well. Most of the time.
Healers in most games are also simultaneously among the highest damage dealers. Lets see...In FF7, if you were using healing materia then your magic automatically goes up. FF9, the healing characters (Garnet, Eiko) were also the Summoning characters. FFX, Yuna was the healer, and once again the summoner (which made her one of the most OP characters in the game once you got Anima). Breath of Fire III, Ryu (the main character) gains nothing but defensive and healing abilities as you level up, but has the exclusive ability to use dragon transform (strongest in the game.) Final Fantasy XII, Ashe and Penelo were probably the best magic users. Their healing was absolutely amazing, but if you turned them around and made them attack with magic they'd outdamage your entire party.
The difference between healing in PSO and in most good RPGs is the time difference, i believe. In PSO, your HP decreases extremely fast, and thus the ability to quickly heal is pretty important. Also, healing items usually restore most of your HP, and are not very difficult to obtain. The reason RPGs require healing characters is because if the non-healing characters were left to heal on their own, they would recover damage slower than they received it
. And most importantly, while the damage dealer is being focused on staying alive, he isn't doing any damage.
And if by chance one of the damage dealers DIE,
well....you have a big problem, because now 1) your enemy has 1 less person to attack, meaning alot more damage is going to the other characters, and 2) he's going to be MUCH harder to keep alive than when he was just low on HP, because once you revive him you still have to bring him out of critical health. The Healing character isn't the only one that's losing damage, it's the entire party.
In PSO, when the damage dealer starts to die, he immediately pops a mate and keeps fighting, probably without interrupting his combo. The only thing Resta does in PSO/PSU is save the other players money/resources and stop them from having to run away, and save them time from having to do it themselves. So, if they want to make classes more relied upon, all they really have to do is balance the amount of time the actions on this game take to carry out. If a Hunter has to flee battle to be able to safely pop a monomate, then the ranger giving him cover fire or the other hunter on his team is going to be very, very useful. But if there is a force nearby, he can get back to health far quicker. And while the force is maintaining the party, inbetween safe periods it can choose to dish out some serious damage.
The force will need both faster (but weaker) and slower (but more powerful) techniques to use in case he is not maintaining the party. A staple of healing characters is that their defense is terrible, which is usually because they have a lower chance of being targeted because you can put them BEHIND other characters. Therefore, they usually are able to end the battle in a single attack if left to themselves. So yeah, the issue really is timing. Healing items and Magic don't need COOLDOWN times, but they most definitely need Warmup periods. This not only makes them risky to use, but it makes you respect the fact that others can do it better/faster than you. Shifta/Deband/Zalure/Jellen should definitely be Force specific gimmicks, too.
Aaaannnddd finally, the biggest mistake that MMORPG developers make is that they base the ENTIRE
game around the stats of the characters
. You are COMPLETELY allowed to let a Force character nuke an entire field of enemies, but only if it doesn't work throughout the entire game. Perhaps their magic defense is a bit higher, and it would be a waste of TP to use powerhouse techniques, forcing you to debuff them and use faster ones. They need to make sure the ENEMIES are unique enough so that you have to switch up your strategy. PSO was able to achieve this by making the different elemental abilities actually function differently, so you are forced to fight different element enemies in different ways. But there are many ways they could do it.