Fireball does 8D6, meaning that against four targets you're looking at 32D6 damage, half on a save, so let's assume half failures and say 24D6. For Hex to do that much damage at level 5 (spell level 3), you'd have to get 24 hits in 8 hours. At a maximum of two hits per turn at that level, and considering concentration can break, and considering you might need to concentrate on something else...well, call me skeptical.
On the other hand, if you have an extra spell slot at the beginning of a short rest, you can Hex yourself and maintain it throughout, then apply it in the next combat without losing anything. That's a good use.