It arguably is prevalent.
I think it's more an anecdotal thing, though. As someone else said, if you're a generally good player, you're less likely to see it. You'd have to run a bunch of content (random stuff, not stuff you pre-decided with friends) only healing and not casting any damage spells to see if it happens or not.
This is REALLY anecdotal stuff. I often que for things as the three roles, and I find tank ques instant in 4 and 8 man content, healer ques instant in 4 man and some 8 man content, and dps ques instant in 24 man content and sometimes 8 man content. Indeed, I did a lot of Pande normals on DPS or tanks since the healer ques were the longest when it was fresh content.
I've yet to wait in any que for more than 30 sec as a tank, and I've often had to wait that long on healer. So this idea that healer ques are all instant and tank ques aren't is really strange to me since tank ques have been instant every time I've done them. So I genuinely "Press X to doubt..." that tanks have these wait times that healers don't, since my experience has been the two as comparable in most respects with the difference being more on the party configurations of the content.
Again, using what metric? What objective - that is, not anecdotal - metric do you have to support this position?
The Lucky Bancho numbers go both ways, and are unofficial, and we have...literally nothing else to go by. In general, that is, across all types of MMO and MMO-like games, tanks are the least played role, though it's close enough that the two can sometimes be roughly comparable in games. Further, you specifically said "by far the least played", which I highly doubt. I doubt that healers are less played than tanks in general, mind you, but I suspect that if healers are less played, it's by a small margin.
Especially since you said "in casual content", which is the content where people (casual players) seem to actually enjoy the role. If you meant "healer is by far the least played role by high end players in casual content", that might be more accurate. Though I'd still question the "by far" of that statement.
But in general casual content played by casual players? Casual players seem to like healers right now since they're very veg friendly.