G'day TP,
Until this conversation I'd not considered Rad Alt as a source, probably because for 20 odd years I've used VSI plus Altimeter, so I wouldn't be brave enough to suggest Airbus (no bus time personally) has it wrong.
However, much like an engine failure where two instruments should be used to identify the failed engine, I believe it unwise to rely on only one source for positive rate. I guess my main bone of contention, is being asked to retract the gear as the mains leave the ground with no thought of what might happen should the aircraft encounter a downdraught or for whatever reason not really be climbing away. Sort of like the occasional incidents of folks selecting gear up on the ground and relying on the squat switch to keep the gear from retracting (admittedly this is usually associated with light aircraft).
In the 717 FCOM, there's no explanation as to why Boeing recommends the PNF observe positive rate, call it, then the PF confirms same before calling for gear up. But were I a betting man, I'd reckon that at least one reason is to slow the process of gear retraction just a little, to ensure the aircraft is properly gaining altitude, and doesn't settle back onto the runway on its belly.