Hi Howard,
Full hard bodies are good too but there are obvious advantages of a poptop with regards to wind resistance, clearance from tree damage in the bush and some may want to ship in a container which is then possible......but one of the best advantages is the extra ventilation available in hot weather.....by the way, ours aren't canvas. They are ripstop PVC same as ther curtain-sided semis run but two layers sandwiching about 4 mm thick foam padding....so they are insulating somewhat and very weatherproof.
A full hard body would almost certainly require air-conditioning in hotter weather. Not saying one over the other cause we have built plenty of both .....just stating the advantages of the poptop IMO. With a hardtop , the advantages are better thermal insulation, structural integrity of the body, no lifting mechanisms, and you could say probably more internal storage.
Re: Slide-outs. They are great on vehicles that never leave the blacktop.......but corrugations (washboard roads) are a killer on the mechanisms that are commonly available and dust ingress around the slideout panels would be almost a certainty....so that's why you don't commonly see them on proper offroad motorhomes.