Is it fun?

[Rilifane]

I would say the jobs being made so similar now, especially healers, is a result of people trying to force a meta comp in the past for other content. Even now people change jobs every patch almost to try and be the highest DPS ing healer. The fact that people do change jobs so readily is also an indicator that the jobs themselves aren't all the fun or unique, since people are so willing to drop one and play another. Most healers I have seen here keep all 3 max level and do so because they know they might have to change to another if one job has a lead on another, again no matter how slight.

I don't doubt that some people do that but what I usually see is that healer mains play all 3 healers simply because they're healer mains, want all on 80 and a general overview of what they can do. And they quickly get bored of one healer and end up switching it up every few weeks/ months with most gravitating towards WHM even though it's currently the weakest (again).
Many dps mains also tend to level several to max because they like having options but they usually stick with their preferred one longer even if it's currently not the strongest. There is still some fluctuation towards the stronger ones, like everyone and their grandma played SMN last tier and some PFs were even locked to SMN for caster. But several others had the highest number of parses throughout the entire tier even though they were weaker than others, e.g. DRG was behind NIN until the most recent buff and was still more popular. BLM is ahead of SMN and RDM but both get played far more often.
Dps jobs are unique enough to make people stick with them even through bad times, healers are not. So they don't care enough about their class to stick with it and simply chose whatever is still left, fits their current mood, is the strongest or is the easiest to handle.