Variety, like others have said, is a big factor in being in multiple leagues. You can be in different types of league (different scoring or rosters, etc.) or maybe just leagues with different owners (an online league with i-friend, a work league, a family league, etc.).
For me personally I have noticed that when I am in more than 2-3 leagues, the fun level dips for me. 2-3 leagues seems to be the sweet spot for me.
At some point, it comes down to the economic law of diminishing rutruns. You might greatly enjoy your first slice of pizza, and even your second and third. But eventually, maybe on slice 4 or maybe on slice 10, you're not enjoying the pizza nearly as much as you were during your first couple slices. When you hit that point, you have to ask, "Is it worth my money to buy 10 slices of pizza when I am not really satisfied them after I have 4-5? I'm just too full at that point."
It's similar with FF, imo. At some point, there is a number of leagues for each of us that it simply becomes not worth our time, efforts and possibly $. This number varies from person to person, but we all hit a number of leagues where what we get out of adding another league just isn't worth our investment.