I'll give it a shot.
In my league we are allowed to keep 7 players at the end of each year.
We have to keep 3 hitters & 3 pitchers. The 7th player can come from either.
At the start of each year, you draft to fill out the rest of your roster.
The commisher of my league keeps track of everything. He has a small website and posts our information to keep track from season to season.
Positives: It gives you more "ownership" of the players. You see a lot of trades on here..maybe Vlad+ for Abreu+. Well in a keeper, this is a BIG decision, as both will be the cornerstone of a team for years to come.
Negative: If the other owners suck...well you don't get rid of them every year. I have this one guy in my league who always offers annoying & crappy trade offers.
I think you should give it a try. Look for one advertising on here. Just join a free one first.