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While I do level a bunch of jobs (I have a few at 90 with more to come) I play exclusively WHM unless I'm leveling another job. I do everything in the game with WHM and have no issues.
Everyone is going to say the same thing, as evidenced in this thread. Play whatever you want. No one will care if you only play one job or role.
You will miss out on job story quests, some achievements and the understanding of how other roles play and what each job brings to the table. This is a personal choice, no one expect you to level up multiple jobs if you just want to focus on one.
As a few others have already said, you won't really miss out on a lot of game play only content if you stick to one or two roles, but you will absolutely miss out on a fair bit of story so it really depends on exactly what in FF14 you value more.
Of course, given how all of that extra story content is technically optional it does mean that it probably will rarely be of any extreme importance in the main story in terms of basic progression and understanding.
It's perfectly valid to only take on the roles that you feel comfortable with. You will miss out on some things, especially as SE doesn't seem to be moving away from the role quest idea. But nothing says you can't level up something just for those and never touch them again if you really want to do them all.
I level all 90 (note that this just an alt) so I don't have to carry random leveling gear... And I feel like that's my freedom, more armoire chest space. :)