That's a tough decision. I think what you have is pretty much ideal. However, verticals do look terrific on a multisync, especially if you use Soft15khz which let's you add some additional custom resolutions for 300 and 320 line vertical shooters.
A couple benefits I can think of using a digital multisync (I wouldn't bother with an analog one which seems to be practically all that's available these days):
- medium resolution games
- Arcade cabinet-friendly PC games requiring higher resolutions
- keeps geometry settings between games
Another thing to consider is how much longer digital multisyncs will be around. I would bet in a year or two there will be no more digital multisync CRT arcade monitors available. The D9400 is pretty much the only one left.

If I could afford it, I would buy one as a backup for my Betson.