Isn't it possible that that particular bit of advice was specifically for Betts, and not a general philosophy I mean, Betts is blocked at the major league level (yeah... getting way ahead of ourselves here) by Dustin Pedroia. He has no real experience at third, and apparently doesn't have the arm to play short. So if he's going to end up on the major league roster, he's going to need to impress at the plate in a big way. That means he'll need to do more than maintain a high OBP. He doesn't appear to be well suited to utility duties, so he may have to move to the outfield if he's going to be a starter for this organization. His window is unfortunately timed because of the Pedroia extension. Dustin isn't going anywhere, so Betts needs to keep hitting for the power he showed this year if he's going to become a legitimate corner outfield option.
Honestly, I could see him becoming sort of a Ben Zobrist type. Someone who spends a lot of time in the outfield but backs up second base as Pedroia gets older and maybe can't handle 150+ games a year anymore. In that light, being more aggressive might be the right advice for him.