i saw a band practice once, many years agone, and i knew they sounded terrible but couldn't work out why. turned out one guitarist was tuned to E and the other to E flat! :oops: and it took them three songs to notice!
i think a lot of guitarists start out not putting enough emphasis on being in tune and then that habit stays with them. i see so many guitarists playing in pubs where i can say straight away, the G string (no jokes Sambo) is out, sometimes the high E, but they play on seemingly oblivious. the thing is i sometimes do it myself, especially on a certain guitar which, once the strings get a bit old, the G is difficult to get right. and i do most of my practicing unplugged, half a house away from any of my gear.
i'm rambling but i do have a point! i recently got a tuner which clips onto the headstock and tunes by vibration. it's great! i generally keep my guitars in tune by ear, but gradually they do seem to tend to end up being a quarter to half tone flat, so for a quick check up, or for the odd troublesome string, this wee tuner's great. definately my handiest bit or piece!
i can see applications for the auto tuners; mid gig or even song changes to open tunings has been mentioned. but generally i think they're a bit overkill. has anyone ever seen Richard Thompson? sitting effortlessly retuning to some open tuning or other while carrying on a conversation! maybe if they cloned him, and made him tiny we could all carry one in our gig bags, a kind of tuning gnome...