A quick-change capo is an essential accessory for DADGAD playing , because you can't (well, shouldn't) just play in D all the time. It's practical up to seventh fret (A). The only common key that just isn't available (except with a very limited fingering range, and the guitar sounds exceedinly high-strung) is C.

Conversely, though, if you work out how to play in different keys, (like I-IV-V in G, no capo) it has an interesting, edgy sound that doesn't resolve because the chord inversions aren't what you expect. Which can be kind of nice when you get tired of the excessively complete, harmonic sound of DADGAD in D.

I got a nice broody effect out of playing in A, no capo, on my fingerpicking arrangement of the tune "Skibbeeen Lasses". (Thats not the way its done on my MP3 page though )