Simple solution: Everybody level a mage job. No matter what fight, you will ALWAYS need healers... so if you have one leveled, voila. You avoid the inability to join said parties.

if you DON'T want to level the job other people want you to be, then don't expect to do the content with them. Either make your own group, find a group who will take you, or whatever... But in the end, people would rather have the highest chance of winning... and there is almost always a certain setup that gives you an edge of winning any fight.