I was having a pint with some friends from climbing the other day at the Granite Brew Pub and one of them asked me why anyone would want flavoured beer. Like isn't a normal, finely crafted beer enough? Don't random flavours ruin the beer? The context was in regards to one of the beers Granite had on cask that night: "The Chai Wallah Has A Moustache" which is an oatmeal stout flavoured with chai.
I didn't have a good answer for him that night other than to make the distinction between the major types of flavoured beers. Something like Bud Light Lime is really just Bud Light with lime-aid mixed in. Flavoured beers from the craft beer scene tend to use high quality ingredients for the flavours and those ingredients are often mixed into the mash that is the precursor to beer. The brewing process then continues with the extra ingredients so they can become more fully incorporated into the final result.
Flavours have been added to beer for as long as beer has existed, which is a rather long time. I recently had a Scotch ale which was brewed in a style that is over 4000 years old. It involves the addition of flowers from heather bushes after boiling, which are left in the mix for an hour to infuse the beer with flavour (almost like tea).
I think beer can be a great vehicle for exploring interesting flavour combinations. I still prefer to drink beers that are more "pure" in style, but I also do not hesitate to try some of the more random things. The chai beer I mentioned above has a beautiful aroma like chai tea, but the flavour is mostly standard stout (a good stout, but a bit of a tease given the nose). I don't normally like fruit-flavoured beers (I find them too sweet), but spices, chocolate or coffee can all go very well when combined with beer.
At Cask Days this year I had a spiced pumpkin ale which was amazing, and I recently picked up two bottles of Vanilla Bean Imperial Espresso Stout from Great Lakes Brewery. This is one of the best flavoured beers I've ever had. It isn't heavy-handed with the flavours, they simply accentuate the natural taste of a very fine beer. Too bad it was a just a one-off; it's unlikely they'll make more.
So why would people want flavoured beer? If it's something like Bud Light Lime or Coors Light Iced T, I have no idea. But I don't know why you'd drink the unflavoured versions of those beers either. With craft beer though, you can get some fantastic combinations and flavour profiles that are just totally unique, unexpected and occasionally awesome. So although the majority of my beers are "normal" I will still continue to seek out and sample new flavoured beers whenever I can.