I think there is a minor boost especially for good base runners and not just fast ones (Rojas is both). The problem with base running, speed, and stolen bases is that it has value, but it isn't a huge value per action proposition, so it can enhance a player's value, but we are talking more in the half to full win range depending on the position. That might move a player from fringe regular to regular or just tack on to their overall value. It also is a skill that is reliant on other skills, notably getting on base, so the value is limited by the other skills the player possesses. That means being a good base runner is not a carrying tool, it is more a skill that can add value to a player that is already possessing major league skills.
That is a long way of saying, I think it adds value, but on an individual level is unlikely to dramatically change the value proposition of certain players.
