The cleric aura mentioned in the Players Handbook says to see the spell "Detect Evil". I pretty much just change the spells area of effect from CONE to AURA. Since no duration is mentioned I can assume that it's a constant and ongoing effect. Just leave everything else as is. As to what good it is, there are a number of instances I can see it being useful. Lastai's Caress a spell from the Book of Exalted Deeds (p.102), only works if you KNOW the target is evil. Taking a moment to study a suspicious character could determine if they are evil or just look shady. Finally, if you take the aura business to it's maximum extent, you can further determine if the target of inspection is in opposition to you in the matter of LAW/CHAOS. So you can actually have 2 auras going at once, you merely have to concentrate on someone in aura range to gather a bunch of useful information.