I'm a big fan of them because they give me something to do on U2gigs even when the setlist is otherwise static.
But no, I agree with Cobbler. The best snippets can really complete a great performance. The Resolution and Independence intro to Streets on 26 December 1989 is sublime. Other snippets like So You Want to Be a Rock 'n' Roll Star in Desire, Love Will Tear Us Apart in WOWY, Movin' On Up in ISHFWILF, Dirty Old Town in RTSS, Life During Wartime in Discotheque, and so so many things in Bad (Norwegian Wood a personal favourite) are great. Even songs I'm not so keen about can really be helped by a good snippet - like Stories For Boys in Vertigo. There's a good example of how a snippet also helps give a bit more representation to the band's back catalogue. I'd love a full performance of Mofo, but getting a few lines from it at the start of Iris is better than nothing.
I also love it when he snippets something really random and silly, like that time he did Come On Eileen in Sunday Bloody Sunday.
But when it's just random shit that doesn't fit, or when he starts to do something and then stops, it's not so great. It's especially annoying when it's being used in lieu of a proper segue - you all know what I mean, Mother And Child Reunion leading into Streets. It's been over a decade now since we've had a decent segue into Streets.