If a manger is forced to keep players received then why would you want to trade? Basically what you are saying is that only a manager's 2 keepers are the only available trade items.
Yes, exactly. You can trade your 2 keepers and your 10 draft picks. Trades are only made to move up in the draft or to upgrade your keepers.
Let's get a few basics down.
There are 12 rounds of 12 picks (144 total picks) in the draft. The first 2 rounds (picks 1-24) are used to assign keepers to their teams. These picks are pre-set and you HAVE to draft the 2 players listed as your keepers.
After that (starting with draft pick #25) any player left on the board is up for grabs. Nobody is any longer forced to pass over players.
So you physically can't have 3 keepers, because there's no guarantee that the said player will be there. Furthermore, you can't trade, say, your 3rd round draft pick for a third keeper, because, again, because there's no way of assuring he gets assigned to your squad.
Therefore, every player must go into the draft with 2 keepers and with 10 draft picks. No ifs, ands or buts.
Now, if I have terry, and I'm not going to keep him, why can't I trade him? Because, then someone whom will have had no effect on your next season will have, in effect, helped you out. There's a reason this league was setup with only 2 keepers, and not with retaining their entire team.
1) So that someone who messes up or doesn't make the best changes isn't doomed for a decade. Forcing you to re-draft means you have to do well year in and year out. We don't want someone whom made a mistake to lose interest because he can't escape the bottom. This isn't the NBA with 4 year rebuilding plans, players should be able to be competitive every year (if they do well).
2) Strategy. Depth v. Keeper quality. Some people will forego depth (and thus a more realistic shot at a championship this year) to get two stud keepers and put them in a position for years to come (with solid drafts and management). For example, it's been no secret that I have wanted a stud guard to pair with KG, but do I want to overpay right now, lose my depth and possibly my shot in the playoffs this year?
If you could use non-keeper players as tradeable assets, you could get the best of both worlds, in both of these issues. Since you're forced to keep a player you receive back in a trade, it prevents this.
Now, since you can't force others to pass over players after round 3 (and therefore can only have a combination of 2 keepers and 10 picks), what trades can be made in the offseason?
picks for other picks. Ex: want to move up in the 2nd round. You trade your 2nd round pick + 3rd round pick for a higher 2nd round pick and a 4th round pick. You move up in the 2nd round but move down into the 4th round.
Keeper for Keeper. Straightforward.
Keeper + pick for Keeper + pick. Terry + 3rd round pick for Pierce + 6th round pick. a player gives up the better keeper (Pierce) to move up 3 rounds in the draft. I get the better keeper but move down in the draft.
But in all trades in the offseason, they must be equal in the number of keepers and picks involved. Any other combination will create discrepancies in one person having too many keepers, which can't happen for the reasons stated above.