As you all know, this spell has a very short duration (compared to chaotic commands). That makes it pretty useless, and you need other means to fight the undead (a mace, an amulet...)
Don't you think it's an error? Who will ever learn this spell?

Priests automatically get it.

It's no mistake. This spell really is supposed to last for 6 seconds/level.

It's long enough for me. Most of the time, by the time the spell runs out, the battle is over. Sure, there are some places where there are copious amount of level draining creatures, but most of the time they occur in small enough groups that you can fight them with just your characters with permanent negative plane protection from an item. For the big battles, if you want to get everyone involved, the spell's duration is long enough. Level 15 lasts for 90 seconds - how many battles last longer than that?

1 round per levels not that bad? What do you need it for more than turn for? Vamps aint that hard to kill...

it only takes around 6 seconds for a sunray
:)

Early in the game, I'm always complaining that it's too short. There's a lot vampires around chapter 2 when you're still weak (ie: crappy turn undead, no sunray yet). Most parties can handle them, so long as you attract all the vampires to hit your protected fighter. But in solo or without someone to "tank" it, then there's trouble.

So early on, I always make sure I have 2 negative plane spells memorized so that I can recast it if needed. It's not hard to find space for the spells either, since Animate Dead and Holy Smite are still relatively weak. Later in the game, when better protection and faster ways to kill undead are available, then I phase the negative plane spells out.

Negative Plane Protection has a fixed duration of five rounds (check your spell description). It's woefully short, but the casting time is only 3 and it can help you in a pinch (e.g. you're suddenly assailed by vampires). If you take it, best put it into your first slot so you don't lose if a vampire gets a hit in.