I guess it's also depending on the Roland model you have with the D-Beam. I find the D-Beam EXTREMELY useful! I use it all the time, I set it to control Reso., Cutoff, volume, and other things. It is a very expressive tool IMO.

You want to have fun with the D-Beam? Pick a good synth bass, something that sounds like an old Juno Bass. Turn the Reso up in the + direction, and set the D-Beam to control the Cutoff. Or select a string voice, like a violin or cello, and set the D-Beam to control the volume. Try that and listen to how much more expressive your string solos can be.