I feel somewhat guilty about slagging off this stop now having been ticked off by JPM but, honestly folks, I've played hundreds of English organs and the Dulcianas are nearly always a bitter disappointment - I'm sure when they're good they're useful, but I've never heard a good one. A much more useful stop I've encountered in a few modern organs is is a Viola, which makes a very useful bridge between the stopped and open diapasons - I suppose that's just a bigger Dulciana, is it?
There are things that one can use Dulcianas for apart from accompanying the Swell oboe - sometimes the Dulciana can add a rather yummy quality to the Swell Celestes or make interesting combinations with the Great 4ft flute.
That said, I'm still not convinced. On a small Great it amazes me that builders have in the past chosen a Dulciana rather than a 4ft flute, which is so much more useful.