It's up to the developers to implement something choosing from the options available. I have described the most common implementation that people ask for. Taming wild monsters to use them in battle.
Things that are unrealistic are unrealistic. You apparently have no idea how design at all works if you equate adding one unique class to adding several unique class, let alone dozens.
And it is not unrealistic to balance tanks. It is straight impossible to perfectly balance tanks without making them literally the same thing with different names, maybe order of skills used for most efficiency. The game works on maths and maths have only ONE result to everything, regardless of the amount of paths to achieve it.
It is possible to balance tanks so long as you consider "viability" as the focal point however. Since viability allows for certain classes to be better or worse in something, they may have varied traits to them while being balanced.
Then don't?! I don't go through the effort to learn most of the jobs in the game. Should they be deleted because they are useless?! So I wouldn't have to level them and forget they exist just cause I'm the sort of person that does tend to level crap I'm never going to use anyway?!
There are plenty enough people that will level Blue Mage and that will enjoy it. It does not need to fit everyone, let alone do that straight out of the box.
If you want to show your opinions of Blue Mage?! Sure, go ahead. But stop attacking opinions of others. By calling it a joke, waste of resources, stupid and the like you ARE attacking opinions of others and going far further than you have the right to in an opinion. Those describe "facts", not "opinions". Opinions focus on the person giving them. "I don't like it", "I won't enjoy it", "I couldn't care less about it", "I'll see nothing from the resources put into it". These are opinions. Not what you and most other people here are showing.