Even if you seed for a very short time is speeds up the process. No need to consume their data.
Only if they seed more than they leech. It has little to do with time. If you take a year to seed more than you leeched, you're a positive contribution to the swarm.
IMHO the most honest comparison is to hosting the entire file yourself.
In a general sense, any video content where the bitrate is lower than the typical users upload bitrate could work for a webtorrent only swarm of 'regular' users who aren't going to do anything special for network health.