J/P's are a bit different with dust collection and don't have as much benefit from "gravity" like the separate tools do. Things can get plugged up very quickly without DC or when one, um...forgets...to open the gate. ("speaking for a friend"...heh...heh...) It's all in how the ports are setup on a combo. I'd be very surprised if any J/P combo manufacturer didn't require DC for proper operation. All of the major brands like Felder/Hammer and SCM/Minimax do for sure.
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The most expensive tool is the one you buy "cheaply" and often...