I've got mixed opinions. Sometimes it's a beautiful communal celebration, echoing a tradition as old as humanity. Other times it's a drunken pillock, honking like a pissed goose, ruining a transcendental moment.
My nephew, whose singing skills could at best be called enthusiastic, got told in no uncertain terms, to stffu at a Michael Kiwaknacker gig a few years ago. I love the boy with all my heart and I'm not a mad fan of Kiwaknacker but I agreed wholeheartedly with the complainant.
Otoh, I've been party to some excellent, uplifting singalongs; Stiff Little Fingers' At the Edge is always a beautiful choral thing, despite/due to being usually shouted by hundreds of pissed old punks; Leonard Cohen's Hallelujah sung by thousands of us on a warm summer evening; Dylan's Like a Rolling Stone rendered perfectly by a festival crowd while the curmudgeon did his own croaking version which we rightly ignored; Macca sending tens of thousands of us into the night at Glastonbury, all still doing the la la las from Hey Jude (simultaneously toecurling and gorgeous); Resurrection being sung broadly in tune by loads of mashed up 50+ dads, correcting Ian Brown's braying...