Jack, I suppose the question is what do you want to play on it? & Who with?
A/D boxes are very low-pitched, being just one tone up from a G/C, which is the lowest. They are used by a lot of continental players as an alternative to G/C, partly because some people prefer the slightly higher pitch, and are generally played in the upper octave. I don't know if they have any more specific uses.
Obviously you can play them down the bottom as well, and if you want to play a lot in A they are very useful. I have an A/D Erica which I bought very cheap on Ebay just for that purpose.
Most players in this country play a lot in G, though, which is possible but not straightforward on an A/D (it's like playing in C on a D/G). If you want to play in sessions and play the usual repertoire of British tunes then you really need a D/G as a first priority. I think A/Ds are a lovely pitch, though, and if it's a nice box and cheap enough you would probably enjoy playing it, but you'd probably end up getting a D/G as well....and hence taking the first step on the slippery slope to MADness!