Not rare at all if you sit on a train and glance around you. I see people watching movies/TV on phones and tablets constantly out in public. Very common.

It's partly the reason I got a Fold 3. Very good for on the move media consumption.

As for data use, aside from the points already brought up - ability to download for later from Netflix, certain services not counting towards data usage on some plans, etc - Netflix and other streaming services will also know you are on a mobile network and provide a more compressed stream at a lower resolution (720p with lower encoding settings being common) to reduce data usage and buffering.