Absolutely nothing wrong with using a capo, ever. There was a discussion about this here not too long ago, and nine out of ten guitarists agree, the capo is a-ok.

Last night, I was playing some songs by the French chansonniere Barbara, which are often in flat keys, or rather, have sections in flat keys, because her songs usually contain key changes. I was playing in such a way that I was holding down a lot of chords, and it was murder on my left hand, because there was no possibility of using an easy chord in there somewhere to give it a rest. In such situations, using a capo is a good idea.

The keys in which the songs are published are impossible for me, because they almost all go too high, and I don't have the voice to sing them, anyway. I was too lazy and impatient to transpose them.