When you start talking about speaker wire and gauge, you have to talk about distance too. If you've ever put together a PA, you know the longer the cable is, the fatter it needs to be to carry the current. The 6 inch or less wire inside your amp isn't choking the current because it's short and right up front. If you ran 3 feet of that down to your cab, though, you'd have a problem. The wire to your cab and inside your cab needs to be fatter because of the distance it's running. The leads to the speaker don't need to be that fat because they aren't running a long distance. It's the distance of the cable run that matters the most when you're talking about speaker gauge.
If you're running 10g cable inside your cab, it's not going to be any different from 14-12g cable because it isn't a long distance. Now, if you're running a cab 20ft from your head on the other side of the stage, then a 10g cable is better than a 14g cable. It's the length of the cable that requires a larger gauge.
From the literature I've read, there is no consensus about specific gauges vs distance. Because it all depends on the signal feeding the line. But for instrument speakers, many sources say something around 16 AWG for distances less than 50 feet. Plus, I wonder how many people outside of regular touring musicians playing stadium shows have to run a speaker cable from their amp to their cab more than 3-6 feet. If it's something like a run to the sound board or front-of-house, I doubt a guitarist in a band is going to have to supply his own cable. Really, I'm just concerned with amp -> cab. And for the most part, a 1-3 foot cable will do just fine. I see little need for those high-end, luxury cables touting oxygen-free adamentium platinum hexagonally braided wire is all. And I tend to stick to gauges that are reasonable. But people do what people do I guess. 
