I never saw (or see) the need to combine them. Early 2000s I figured in-ear phones (headset minus the phone) would be a thing Real Soon(tm), and then a PDA would be all I need to be productive. I still miss Palm and the apps available on it. Naturally, there is benefit to a connected PDA, but cellular? I wouldn't miss it, if my phone was just my headset.
Any else have a PDA and see the glaring opportunity to add cellular functionality to them?