scaff tube, or 'blue band' as it is sometimes known is actually a fairly high spec material as its classed as construction grade... in theory it would be good for a cage as it is considerably stronger than some tubing used by some companies. however, due to its considerable wall thickness it is extreamly heavy and a bit of a sod to pull around a tight radius.
if i was just building a roll hoop with a couple of stays i'd consider it, but for a full cage i reckon it'll be a bit heavy.
for those who think it might not be strong enough, when you next see a building covered in scaff, think how much weight the lower tubes are supporting... in relation to the weight of your 'rover...