The ability to pick up a tune by ear is great IF ITS ACCURATE

With folk, even that probably depends and can depend even when people are playing together.

In a session, while there may be versions of tunes that are so far away from each other they do not fit I don't think too many would consider an odd note here or there (if noticed) of much importance - after all, we may all try to play our own twiddly bits putting in what we can or omitting depending on what we feel capable of, how the tune is going at the time, etc.

What IMO, is more important is the ability to fit in with time and hit the beat when others are and, at least in the ones I go to, to be flexible with that so you can follow (and keep to) the tune how whoever started it is accenting it. (I suppose like a jazz "swing it" but rather than having a set "swing rule" it's set by the starter or perhaps in some cases could be refined by a style, eg. Claire - although with as an I suppose "generic Irish session player", these finer points are beyond me).

Lots of different ways and rules, perhaps some more suited to dots than others? As I said in my first post, I do believe that ideally one can do both.
But I wonder whether some of the sort of can't do without dont's are really loking at things from an only been taught/; only have learned a from of music through dots perspective?