Crawler's 'Sticky Thread' attacks
Haste is a better choice for this. It cancels the slow and gives haste. The only time erase is needed to counter slow is from spiders.
As a blm, you won't need it much. Wait until your blm is 64+ and see if you have spare money. If you do, go for it, otherwise don't sweat. The spell is used very rarely. The only aoe's you run into that need it are bats and crabs. You only exp on bats in garliege, so not a factor for a 64+ blm. For crabs' str down effect, the return on time spent casting, spent mp, and time missed resting that could be put towards more nukes pretty much outweighs casting erase on everyone. For a blm erase is a toy that gets little use. For a whm, its a tool that gets very dusty between uses. Put just about all equipment and other spells before erase.