Open mic nights do vary - some do indeed need a PA for performers to be heard over the background noise; but others simply provide needed amplification for solid guitars and quiet singers. I occasionally dive into one, but forego the amplification - the fiddle being quite loud enough (some may say TOO loud) without it, and in any caes a mic on a stand is pretty useless to a fiddle, as you need to stand like a statue to keep the sound level constant.
Don't knock them - it's more important that people just get out there in the community and play, than that they play "traditional" music. And by joining in we can make folk less exclusive.
A few years ago at Tenterden in a session, a number of youths in the pub were not overly impressed; "Can you play Happy Birthday". We duly obliged, and they became a good audience.