I don't find it ridiculous at all. Like I told you in Limsa, it will likely be impossible to balance without stripping the class of what made it unique. Also it also stops the headaches when Mr. Blue Mage shows up in an end game dungeon still missing most of his spell book. And you can't say "Well they have to have so many spells to be able to enter." Ok but what if they show up with the most useless spells Blue has available? Say they have to have these specific spells? Then you may as well have Blue learn spells like every other class does, which would strip Blue of the unique flavor it's always had.
Would it be nice to see it being able to be used, sure. I have friends who would like to main it. Is it possible, eh, I'm no game dev...but I'm sure SE tried all avenues before deciding to limit the job. It preserves the true essence of what Blue Mage is without letting it inhibit the gameplay of other players. Also, not like you can't get some buddies and do the dungeons with them.