I personally don't mind if dancer is a DPS or healer, I'd just like a new healer in 5.0 XD I'd prefer Chemist but I don't mind the thought of dancer being implemented as a healer either. As others have said, both jobs have the potential to bring something a little different to the healing role (the most common idea being that they could be non robe wearing non casters). Either way, as the OP said, the leaker has been right about basically everything else so Dancer is a fairly safe assumption and I personally feel the assumption that it'll be a healer is also a fairly safe bet.
And, to add to whether a job is in a specific role matters or not, it does and it doesn't. It really depends on the player. I don't mind branching out into tank and DPS but I almost always play healers (just personal preference really). As such, when a job i prefer gets added to DPS I know instantly that I won't be playing it as much as I'd like. On the other side, there is the fact that this game does allow you to play anything on any character and that does mean that if you simply like the aesthetic, the role doesn't matter (and as someone else said in this thread, switching roles is the true variety in the gameplay). I know people who switch roles far more often than I do but I also know people who just stick with 1 or 2 jobs (both in the same role). That's why it does and doesn't matter which role a job gets added to (but seeing as it does and doesn't I'd lean towards the fact that it does).
Just my thoughts =)