Hypersmurf said:
[T]he Evil descriptor tells us that a/ druids and clerics who are Good or have Good deities cannot cast the spell, and b/ clerics with the Evil domain cast the spell at +1 caster level, and c/ the spell will be flagged by a Detect Evil spell. That's about it.
Hypersmurf said:
Fire vs Cold is not prohibited to a generic cleric the way Good vs Evil is.
And wouldn't the same principle apply to your Cold worshipper if he cast, say, Prismatic Spray - a spell that deals fire damage without the [Fire] descriptor? Shouldn't the deity be just as mad at him as for summoning an Azer?
There is a Platonic dialogue called The Euthyphro, in which Socrates debates the following question: Taking it for granted that God loves the good, does something become good in virtue of God loving it, or rather is it the case that goodness is logically prior to God's love, and God discriminates in favour of the good?
Socrates, and most philosophers since, answer in favour of the second option: goodness is logically independent of God's love, and God loves the good because of their goodness. The only major philosopher I know of who answers in favour of the first option is Hobbes, who takes the view that God's love is inscrutable, but he loves certain things, and in virtue of that we call them good, and better love them also if we are not to get smited by God!
I bring up this debate, because it seems relevant to Hypersmurf's comments quoted above. Bringing things back to D&D, why do Good gods forbid their clerics from using [Evil] spells? One natural answer is similar to Socrates answer above: because use of [Evil] spells is Evil, and Good gods forbid their clerics from doing Evil. This suggests an answer of "Yes" to the original question.
If Hypersmurf is correct, however, then the above thought is wrong, because the correct answer to the original question is "No". The reasoning, therefore, must instead go like this: There is a class of spells forbidden by Good gods to their clerics, and we label these spells [Evil] in virtue of this fact. Suppose we then ask, Why do the Good gods forbid these spells? As with Hobbes' treatment of the Euthyphro argument, no answer can be given. The Good gods have their own inscrutable preferences, by which their clerics are bound.
It is a reason in favour of the "Yes" answer - perhaps a weak reason, but a reason nevertheless - that it makes the motivations of the Good gods explicable rather than inscrutable.
Of course, as others have pointed out, a "Yes" answer also suggests that the [Evil] descriptor has not been applied as thoroughly and consistently as it could be. No roleplaying rules set is perfect, after all.