I have to disagree.
Speaking from my own experience, I got my team right around the All Star Break. If players off the TL weren't allowed in the playoffs, then there would be little incentive to even try to improve the team for half a season. By that time, you've lost the new player--he's decided it's not worth the effort, gone on to other things, and who knows when he'll come back to the game?
I understand the frustration of those who have tried to manage a team well all season and suddenly have to face a team that is now not a #4 seed but a monster due to late additions, but some of these responses go too far imho.
I do agree with the idea of moving the deadline for playoff eligibility to 2-3 weeks prior to the end of the regular season. By that time, a brand new owner won't have to wait as long to reset their team (their team's playoff fate is probably already an inherited situation), but it forces the tankers to have to pay the exorbitant mutant salary for a few extra weeks. I also agree with some kind of GS adjustment, either for the new transfer, the team as a whole, or both.
I'd also suggest new transfers come in with "Mediocre" GS and the team takes a 2-3 point hit to their Enthusiasm to simulate the adjustment period of getting used to a new teammate. I think these three measures would provide enough of a disincentive to provide some sort of balance to the other playoff qualifiers.
Just my $0.02