The big difference is that multiple SIMs in standby mode don't require any additional chips. It basically comes (nearly) for free.
That's why dual-SIM is an absolute standard feature on nearly every small less known manufacturer.

(Though prices of crappy cams is falling, to the point where adding extra doesn't double the price.
I still remember a time when putting the single cam on a swivel (eg.: Sony Clie) was cheaper than adding a second one for selfies.)