One more good reason to keep your left arm straight on a golf shot: precision.
In baseball, a long-term batting average of .300 is pretty darn good. That's slightly less than one in three at-bats resulting in a batted ball good enough to get the player on base without some error by the defending team. Additionally, out of however many pitches a batter will see in a season (let's say they see an average of five per at-bat, an average of 4 at-bats in a game, times 162 regular-season games, so somewhere in the neighborhood of 3,200 pitches per season), the home-run record is 73 in a regular season, and that guy admitted to steroid use. So in a record-setting performance, a baseball player will hit about 2% of the pitches he sees over the wall, and will hit a home run in just over 1 out of every 10 at-bats.
A golfer who hits the same percentage of "good shots" (up in the air and in the general direction the player intended), even using the higher one-in-ten statistic to make up for the fact that the ball isn't moving, or even the one-in-three statistic just to be exceedingly generous regarding the number of variables inherent in the batters job versus the golfer's, would be asked to leave the course after three holes, for tearing up the fairways and slowing other players. The best baseball players can do is simply nowhere near good enough in terms of ball striking precision to even show up at the golf course. The golfer, on every shot, is expected to hit a ball 1.625" in diameter using a club with a sweet spot about as big as the ball itself, on the end of a shaft that can be up to 6 inches longer than a baseball bat. Not only do they have to do that, but every shot is expected to be accurate to the level of Babe Ruth's famous called shot (within a section of bleachers), otherwise it's seen as a miss.
That is the level of precision professional golfers require just to end up with a par score, much less to find the extra 10-15 shots over 4 rounds of golf needed to win a tournament. Their technique, therefore, requires precision over power. A straight left arm keeps the swing closer to a single plane, and more importantly, brings the clubhead right back to where it was at address, which is the biggest aid to precision that a golfer can give himself.