ECN Forum Forums General Topics of the Trade General Discussion Area #12 wire Minimum?

According to the NEC tables I have here (1970s), #14 is rated 15A max. and #12 is 20A.

I assume that you would always use #12 on a 20A branch. So long as length (& therefore voltage drop) isn't a problem, what's the objection to #14 on a 15A circuit?