Jeff is correct, but also consider that with a J/P combo, you don't have the option for a "dust chute" under the cutter head like you do with a typical jointer-only design. The planer table function is in that particular place. Further, the dust hoods on these wider machines spread out in such a way to cover the width that without the air movement that a DC brings, chips can easily get impacted in the hood entry. Remember, that space has to be calculated to be about the same area as the typical 120mm port carries and for that reason, it gets pretty narrow right at the mouth. If you make a little cut with the jointer, you're not really going to get in trouble, but any meaningful use of the machine really does require dust extraction.
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The most expensive tool is the one you buy "cheaply" and often...