Playing tunes is entirely different. Even if you can play with the appropriate style while sight-reading, and whether or not you learn tunes from dots, to participate in a session then sooner or later you have to set the music aside and play by ear. When a tune starts up, readers have to identify the tune, find the right page (and perhaps the right folder) and hope they have the same version in the same key. By the time they are ready to play the set has moved on to the next tune. Hence the need for "slow sessions" which I mentioned earlier.