The problem is defined in two aspects. Firstly if there are more and noisier accompanying devices that drown out the lead instrument the tune is lost... if loud players of eggs, spoons, borans (and the like) play so loudly or are in such number that it becomes impossible to hear the tunes and/or the notes that are being played the session becomes somewhat pointless... One night I sat in a session with a semi-circle of 6 "drummers" to my right. I could not hear the fiddle player sat next to the concertina player immediately on my left. The fiddle player is not known as a retiring violet. There were 6 differet drumming timings, needless to say the only decent boran player had put his instrument away and gone into the other bar. This guy, now dead, could make a boran "talk" he said to me that there was no way that he could play with "that lot thumping away." I might also include in this group the guitar players who spend the evening thumping chords, usually as loud as they can, presumably in an attempt to get some recognition i.e. qantity out weighs quality, thus drowning out the lead intruments.
The second aspect is quite simply bloody awful timing. I have known players of lead instruments be reluctant to play when egg players etc have such bad timing that it prevents them, the lead instrument from keeping time or playing with the intonation that they would otherwise put into their music.
With all sympathy to those who say "we just want to join in" if you are so "off-putting" that lead instrument players decide to go elsewhere to play see how long the session will last without them.
I have yet to find a session that works on two eggs, a boran and a set of spoons.