Useless?
Melf's Acid Arrow
There is no situation in any game where there is not a better alternative to this spell. It can be negated by acid resistance 5, ffs.
Nope, merely situational. I'm not up on Pathfinder spells, but a quick look suggests 2nd level spells with a range of Long are pretty rare there as well.
In 3.5...
Target is more than 130' away. It needs to die. You are a 3rd level wizard with evocation banned.
Your choices are Melf's, Ice Knife (which doesn't appear to even exist in PF), or breaking out the xbow.
Ice Knife is non-core, which matters in some games. Ranged attack, with a +2 bonus. 9 cold damage, and Fort save vs -2 Dex.
Melf's is core, hits ranged touch instead, does 10 acid damage (split over 2 rounds), and requires a DC 11-14 (plus spell level if casting) concentration check for actions taken by the target for the next 2 rounds.
Fleeing low-level caster? I'll pick Melfs. People do miss concentration checks at low levels.
Troll (a likely level-appropriate boss). Melfs.
Situations where Ice Knife is better (including better than other shorter ranged spells) are fairly rare, so if I want to spend a slot on a long ranged spell, I'll choose Melf's.
It does rapidly become obsolete as you level up, but it's a decent spell for a lowbie.