We do it. It's a lot of fun and it helps protect teams from injury.
It's not that hard to track. Just make sure the specifics are clear and an email sent to the commish with those specifics approved by both teams. Just made a similar deal for Frank Gore - the draft pick is 2nd round if he average 15 or more touches (carries + receptions) from the week I made the trade until the final week of the regular season. We just add up his touches and divide by the number of touches. 15+, dude gets a 2nd round pick. <15, it's a 3rd rounder instead.
Basically, I requested this condition after making a non-conditional trade for McGahee last season, then getting screwed, as he got hurt a week after the trade.
ETA: The conditional trades really become more prevalent as teams make last-minute trades for the playoff run. It is more costly if you lose a guy for 3-4 weeks, when you really only traded for him for a 4-5 week window. You lose a guy for 3-4 weeks over a 13 or 14 game regular season, not nearly as big a deal.
This is all true, and if you want to keep track of the draft picks, very viable.But the OP isn't asking about conditional draft pick trades.
He's asking about conditional player trades. Which is ridiculous.
Worse, he's asking about conditional player trades where the conditions are not defined at the time of the trade.
I have no clue how baseball allows these things. The player who is traded can actually be the PTBNL! There are 4 players in MLB history who were traded to another team for a PTBNL, and then after being used by the new team, became the PTBNL and were sent back to their own team.
Talk about creating a mechanism for cheating. Unbelievable.
We do allow conditional trades but only when the condition is around a draft pick, and it has to be clearly stated exactly how it is to be determined. Any players who trade teams must do so in a single, immediate transaction at the time the trade is executed.