I've played a wide variety of games and tend to be drawn to healer/defensive philosophies. I think its a personality type thing and I do know that there is a certain type of person who prefer reactionary strategies and/or helping out a team and find that much more fun/effective/comfortable.
Like a lot of people playing Demigod I used to play WoW. I played a Priest right from release and was heavily involved with raiding up until several months after BC came out - at which point I burned out. Of course there were a lot of different elements that contributed but one of them was the fact that if you were a healer thats all you did.
I realise quite a few things have changed for the better for healers in WoW and a lot of that has to do with changes made to make things more dynamic, let healers do good healing and damage, effects etc at the same time. But I'll be @#$%ed if I go back to that game 
For a great many people this philosophy of maximising heals and forsaking everything else is still very prevalent and to a certain extent if you play a character/class/faction/whatever that heals there will be an expectation placed on you by other players to focus on the heals and any deviation from that will have people calling you a noob etc.
Its tough, and you may have to put up with a bunch of narrow minded idiots, but resist the pressure to be a healbot.
I think the important thing to recognise about the mechanics of Demigod is that the power of your character is derived from what you kill. While you can to an extent follow somebody around healing them and soak up xp its nowhere near as effective as hitting things. Without the levels and cash you will not have access to skills/equipment and you will find yourself getting owned.
So it may be a little bit counter intuitive, but you need to do damage so that you can be an effective combat healer. Keywords here: Combat Healer.
It helps to think of heals as helpful damage. Don't see your heal as just a restoration of hit points - view it as a sort of reverse fireball that you can throw around. Then add those numbers to your damage output. Even though you may be doing less damage than other DG's if your heals+damage is higher you are having a much greater impact on the game.
What the healer/dps hybrid offers is extreme flexibility in pushing numbers around. I know from experience in trying to balance pen and paper rpgs as well as watching developers try to balance their online games that healer/dps hybrids are incredibly difficult to balance because of this flexibility
A healer/dps hybrid pushing around 6000 points in either heals or damage is much much much more powerful than a healer pushing around 6000 points of healing or a dps pushing around 6000 points of damage. This is why whenever possible developers tend to avoid hybrids or underpower them and thats why the philosophy of healers only heal, dps only dps' is so deeply embedded in the minds of gamers.
All that said - while Sedna can be played as a healer she is much more effective played as a hybrid. It is very obvious when you look at her skills that she is intended to be a hybrid.
Given that the Forces of Darkness don't have access to Sedna i'm inclined to believe that the healer is not necessary. It is very powerful to have access to healing - but there are many ways in which the raw numbers can be manipulated and DG's other than Sedna do have access to skills/items that provide positive number flow.
So in as much as it may appear that I'm advocating healers being 'necessary' I really don't think thats the case as to varying degrees all DG's appear to be hybrids. I would say Sedna is the DG with the highest percentage of healer to dps.
That got kinda longwinded but I still feel like I havent quite explained what I mean but I should probably stop before its gets to tl;dr.
Edit: On the off chance a Dev reads this and actually gets this far, it would be nice if healing done showed up on the victory screen / had some favor consideration...
