It's hard to feel comfortable with speed and defense in traditional power positions. Not only does it negate the positional scarcity addition we made in a $250 million dollar 2B that can hit when we're just going to cancel that competitive advantage by getting a left fielder that hits like a second baseman.
But when your left fielder steps to the plate with two outs in the bottom of the ninth inning, down by a run, the best outcome you can hope for shouldn't be a double. You want a guy there who can tie it up with one swing. It's why 1.4 WAR from Mark Trumbo feels so much more valuable than a soft-WAR center fielder. These are positions (LF, RF, 1B) where you put guys who can tie the game up with one swing, because several times per year, it happens.
When a guy like Dyson comes to the plate, you have to look at who's up next because he isn't going to tie the game up on his own. That's not the bat I'm slating for LF. No way, no how. Dyson should be CF, Martin 4th OF, or traded.