This spell, and spells like it...but mostly this spell, have become a crutch for the developers.
When Sorcs and frequently wizards are less a DPS class and more a "Spam Firewall here!" class, something has gone horribly wrong.
Don't believe me? Go play a Sorc from level 1 to level 20 without ever picking up firewall or a similar spell. See how frequently you get attacked, ridiculed, thrown out of groups, or simply thought of as stupid. I can think of no other spell with the possible exception of haste, and often not even then" that will result in the "You don't have firewall?" lecture.
And the worse of it is Devs have fed into this playstyle. Because it serves as a bandaid. It lets the devs call Sorcs and frequently wizards a DPS class while allowing them to gloss over the fact that it's not so much DPS as it is a "Spam Firewall here!" class.
And that's just sloppy and poor game design.