Its good to have a mix. Often owners will set their players to be good against right handed pitching as this is what they will see the majority of the time. Having a couple of good lefties as starters can be good although there is no problem with having a starting rotation full of righties.
Where I find lefties are very important is out of the bullpen. I would say you want at least 2 left handed pitchers with good ratings pitching out of the bullpen for situational starts. I have a few guys on my team that rake against righties but then are complete duds against lefties. If its the later innings and someone brings in a left handed pitcher against them, I'm screwed.