I don't think that kilt jackets are left unbuttoned to provide easy access to all sorts of weaponry, or to make it easier to play the pipes (I rather doubt that one person in 100 can actually play the pipes).

Rather, I suspect, it is down to a combination of fashion and technology. The fashion aspect allows one to show off his waistcoat or dirk belt. As far as technology goes, we live in a far warmer environment now than did our friends and family fifty years ago, never mind during the reign of Edward VII or his predecessors. Cars, homes, and places of business all have effective heaters, cheaply keeping everyone toasty warm, or at least comfortable, hence there is less necessity to button up one's kilt jacket or overcoat.