Although I have the Porter-Cable Omnijig, the idea is still the same.
Because the bit is basically cutting through the underside of the material, the dust collection from the router is only sub-par. Most of the debris is dropped from the bottom of the bit. Therefore, dust collection from the bottom of the jig will be much more productive than hoping to suck the dust up through the guide bushing, fingers, and material.
Also, the outboard support for the router is very helpful in keeping the router base flat to the jig. It seems that it would be a very worthy upgrade and something you'll never regret once you start using the DR4.
This is one of the tasks that I don't anticipate using a Festool router for. I have a PC 690 set for the jig without dust collection on the router itself. I rely on the dust extraction from the add-on accessory and use it with my dust collector and 4" tubing.
If the bit is cutting the top of the work (like round-over, dado, profiles. fluting), the Festool dust collection is really great. In the case of dovetail jigs, the built-in dust collection can't hope to draw the chips up through the material efficiently.