I think every job has the possibility to aid in other areas if absolutely needed (yes I know DPS should primarily, well, DPS. XD but there are occasions where minor healing and raising from say, summoner, and the occasional sleep from BLM may save a team from wiping. Yeah I mainly play caster so those are my examples. XD)

In most cases, healers "just healing" is perfectly acceptable, however, that does not mean that player is playing their job to the fullest. There are fights in the game that have some pretty significant dps checks, and if the dps are rocking min Ilvl, a healer might be needed for that extra dps.

This game focuses on a TEAM effort and when a player blames everyone else around him/her because they had to "act outside their class role" that player is only focused on covering their own butt and not the over all success of the team.

I say, judge what you should do by what is happening in your current situation and don't hold so tight to class roles.

that being said, it is perfectly plausible to complete most objectives while holding true to the base formula, but is that really fun?