Psimancer said:
Really? Given the fact that it is a Concentration spell (taking a Standard action each round to maintain) and takes three rounds to identify the source of evil, I have never had a problem…
Exactly. Just flicking it on and off just lets the Paladin know that yes, there might be something evil around here... somewhere...
It works like the spell; 60ft. cone infront of you. 1 Standard action to cast, and Concentration (as Psimancer has said) each round to keep it going.
First round, yup, there's Evil here somewhere. 2nd round, you can actually get specifics on numbers. If the most powerful aura is overwhelming, you get stunned and lose the spell. 3rd round, you can actually point out exact locations and powers of auras. If they aren't in Line of Sight, you can only give a direction (it's, erm, North).
Even then, the Aura might not even be from an actual person anymore. Auras linger, so your Paladin might have just spent three rounds finding a trail from an Evil Outsider (11 HD or more) who happened to walk past this point some three days ago...
And it's still only in that front 90 degree arc. Turn around, and you've suddenly got to start all over again. Move in any direction, and you have to wait those three rounds to cover what you've just been able to reach with the effect.
I've played Paladins before, and sure, Detect evil is handy. But if the DM knows that you need to spend some actual time using the damned power, there are a LOT of ways to trip them up. It's not an automatic Alignment-detecting torch.