Everything in engineering is a tradeoff. The more complicated something is, the more things there are to break. Mounting the cameras on a scanning platform allows for great flexibility and has worked well before, but it also introduces another failure mode. Suppose Cassini had used the scanning platform technique like some previous vehicles (e.g. Voyager). That means they would be able to keep the high gain antenna pointing at the Earth while taking pictures in different directions. What happens if the gimballed camera gets stuck in some unusual position? You'd be back to pointing the entire vehicle to get the images, only this could be more complicated by the unusual camera angle.

Given the large solid state recorders (4 gigabits each), I really don't see a major problem with Cassini the way it was built. There are only a few hours a day where you might have been able to transmit data and take images simultaneously (had they used the scanning platform), and that might've been limited by data transmission rates anyway.

Also, Cassini is more than just a camera platform. There are other sensors onboard to collect data. Some of them, such as the magnetometer, don't require precise pointing. That means there can be times when you transmit data as it is being collected.