The downside, from the one game in FM11 in which I ticked it, is that every season will keep the fixtures order, which depending on what it is for your team, can be a problem.
It was a problem in my Sampdoria game: the fixture was so that my rivals were the ones Inter would fight the next week. So take a loss of a point and Inter going ahead of you, you need to win and them to lose, but the already strong Inter would be playing the teams you had to defeat the week before and that therefore got to them with their morale reduced by your own victory. It was impossible to overtake them, they won every match, the only they didn't were the ones I couldn't either. In the second season I found it happened again, it was losing a few points and repeat the process.