There are times of course in sessions where the tune being played is new to you,a different key to what you usually play ,or a different version to the tune you know.It takes you a little while to sort these things out, so it is usual to either not play at all, or work through it quietly so as not to distract other players.This may require you to put the concertina up to your ear to ensure you have the right notes and then wait for an A or B part to come in. Do not join the tune half way through.
Perhaps we could liken this to other pursuits, so for example, how comfortable would you be climbing the North Face of the Eiger, with someone who was reading the Beginners Guide to Rock Climbing, as he climbed?
Or, how popular would a referee be at a Football or Rugby match, if they were constantly referring to the Rule Book?
The way I see it, bookwork is what you do at home to prepare yourself, so it's not surprising really, when folks who have spent years & years diligently doing their homework, frown on those who apparently couldn't be bothered being better prepared.
I wouldn't dream of dropping in on my local orchestra, in my T Shirt & Jeans & expect to be allowed to just sit in at the end of the line, without Music & just busk along with the symphony.
Or, put another way, would you take a knife to a gun fight?
Frankly, if you are trying to read the dots & learn the tune, as it is being played at a session, then it's highly unlikely that you will be adding anything of any real value to the overall sound. In fact, it is far more likely that you will be to a greater or lesser extent, destroying the sound, at least for those sitting next to you.
As Henrik has already pointed out, all you get in a book is a skeleton, so anyone trying to play the skeleton of a tune they don't even know, at session speed, is probably going to be leaving a few bones out in the process.
Leaving beginners sessions aside, the way I see it, a real session should be the pinnacle of a traditional musicians aspirations, so it's only when they have mastered a number of tunes that they should even be thinking of joining in.
For what it's worth, I've found that, during the past 40 years that I've been going to sessions, musicians with common sense & good manners, always do their homework ..... at home.
When in Rome ......
i agree that it is best not to trample on the tune, but i don't consider it a rule that you shouldn't join a tune halfway through. there are times when it is appropriate to wait to join in and times when it is not. all the best players i know do it both ways.
it all depends on your confidence in the tune (especially in another key), the sound that you will be contributing to (one solo player versus a large group) and the nature of the session overall. for example, if you are at a gig-like session and someone goes into a tune that you need a moment to get grounded in, it might be best to wait for a repeat, especially since it will sound like it is planned.