I'm not sure that's actually true. Pache and Marsh and Cave are all better defensively and if Merrifield is in fact a below-average hitter (as he's been for three years) Cave or any other LH bench bat might be better against RHP. In fact, Cave still has a career .729 OPS (110 OPS+) against RHP that is a tick better than Merrifield (.734, 96 OPS +). And while Cave was much worse than that last season Merrifield wasn't good either (with reverse splits, no less)/
So just the fact that people think Merrifield should be the starting LF if Rojas isn't in CF does prove the point that his name and contract might cause the Phillies to make him the starting LF. I'm hoping that's not the case, because Thomson hasn't really shown those tendencies.
But even without Rojas, every extra AB Merrifeld gets is one where you don't get to challenge Marsh, Pache or even Stott (though I hope not) to become more complete players. Maybe that's okay for one more year on this win-now team, but I don't think playing Marsh more against LHP or playing Pache more against RHP would really hurt the team either.
Obviously if Merrifield tears it up at the plate it's all moot. But you still don't really want him to be the starting LF. That's not his value.