Black Mages stink, Red Mages rock. There. I said it.

When you get down to it, all you get in taking a BM is a few more MP, and NUKE.

Pick a Red Mage and you have higher HP, higher defense, decent attack, and a slew of white magic.

The Black Mages higher MP makes very little difference. Before you get the SOANDSO2 casting items, the MP will only be a help in the peninsula and the Earth Cave. Afterwards, I would rather have an extra healer than 1 more charge of LIT3.

As far as level 8 Black Magic is concerned, you get them too late in the game to be of any real use. The sky tower and the temple of fiends don't pose any real problem that could be solved with a NUKE.

As far as accusations of Red Mage becoming obsolete, they are thoroughly baseless. Red Mage easily keeps up to par in magic no matter where you are. He may end up with less Attack Power than fighter, but at least he's on the radar. Black Mage will never do any sufficient physical damage. If you want to count in the Masamune, you must give Red Mage the same liberties.