IMO it is almost always a bad idea.
Advantages: none. (well, maybe "rain can't blow in", but if you put the sidewall popdoor on the usually-downwind side, and if necessary put a little porch roof over it, rain won't blow in *that* way *either*)
Disadvantages: 1) bedding is constantly lost thru hole, no matter how many sills or whatever you rig up to try to prevent it. 2) the bedding lost thru the hole will, in time, make a muddy soggy mess in the area around the ramp. 3) you lose a HUGE amount of usable floorspace for the chickens to roam, and when people want to do this it is nearly always in a coop that is not overly endowed with floorspace to begin with.
The only time I can see it being at all sensible is if you are working on a TEENSY footprint, with the only run space being underneath the coop, and you simply HAVE no room for a side-access door. Except in that very rare circumstance, do yourself and your chickens a favor, and just build a normal popdoor in the *wall*.
JMHO, good luck, have fun,
Pat