The difficulty I foresee right now is that when it comes to enmity in MMOs, traditionally if you're called upon to heal, you don't also try to do any significant amount of damage. With the way the game is currently set up (to include the unfortunate decision on SE's part to not reward healers with skill points for doing their jobs), anyone who can heal other party members also has a vested interest in doing damage to the target. I certainly hope SE isn't of the impression that parties should be a cluster of players all doing a bit of everything, because I reckon the chaos would make grouping with random players more than a little painful later on.
I think the idea of FFXIV is that just because Conjurer and Thaumaturge have damage dealing abilities and healing abilities doesn't mean you have too, or are meant too, use both. That doesn't mean you CANNOT, or SHOULD NOT do whatever you want however.
I really look at the FFXIV template like UO. There are a BUNCH of skills you can learn, some work well together - some do not. Regardless - you're entitled to set yourself up however you want. If you want to be a walking, indestructible heal-bot - go for it. If you want to be a paper-thin cannon, by all means. If you want to be a weird hybird or focus on support somehow - that's great too.
This will lead to people setting nearly all party-assisting and healing abilities - or going strictly for DD. The flexibility of being able to set your skills will allow people to change or adapt there role 'on the fly' (sort of) based on what is needed. It's going to be important to find out how people are 'set up' before you go out and adventure.
It's going to be up to the players to define how they play their characters - it will not be predefined -as much- (<-- keywords) as by selecting White Mage, Black Mage or Bard (for a loose example.. healer, DD, buffer) - it will fall to individuals to define these boundries somehow.
For me - I'm interested in healing for once (maybe) - so I'm going to pick up every healing skill and advertise myself as such. If the time comes where my Linkshell has 40 healers and we're short on some magical damage - I'll set DD spells and take a shot... (example)
There is the issue of non-rewarded healing spells - but I thought I saw confirmation someone has gained through curing party members, it's just infrequent... which would prevent someone self-spamming cure and AFKing effectively, which is good.