The top three players still available on the free-agent market—Stephen Drew, Kendrys Morales and Ervin Santana—all have draft-pick compensation tied to them that would go away after the June draft. Waiting until then could help each player's value, which is why their respective agents have no problem floating that possibility as a legitimate option.
Even if they were to wait until after Opening Day to sign, they'd be more attractive options because they would no longer be eligible to receive a qualifying offer after the season if they happened to sign a one-year deal. By turning down their respective team's qualifying offer this offseason, whichever team ended up signing them would have to forfeit a draft pick.
As a result, several teams have shied away.
There's a reason why a top free agent signing after the season begins would be unprecedented, though. There are obvious fits now. Those could become more apparent as we approach the start of the season. Other fits could materialize because of an injury or a player who isn't living up to the expectations of the organization.
It's also very likely that generous offers have already been made and some could still be on the table. While players normally trust their agents, who are looking to get their client the best payday possible, they could make the safe decision by taking the best offer and getting back to work. Sitting on the sidelines without a job on Opening Day can't be easy for a competitive athlete.
All three will sign prior to the start of the regular season, in my opinion, and in plenty of time to get into game shape. I've predicted their destinations, along with two trades that I believe will go down before the end of March.