That part of their deliberations already occurred... the leak was after the majority of the justices voted to approve the draft. That's why the final opinion matched so closely to the leaked version.
Even if it's a late-stage draft that's leaked, that comes with its own host of problems.
Imagine a world where drafts are trotted out by lawyers seeking to argue their interpretation of an old opinion. "The Court originally was going to hold something like my opponent suggests, but that bit got deleted, your honor!"
That would be another level of the problem of what we might call "arguing the changelog," using pieces of floor debate transcripts in the legislative history of a statute.
Justice Scalia analogized that to "entering a crowded room at a cocktail party, and looking over the heads of the guests for your friends."
And that's a concern on top of the potential for a chilling effect on the judiciary.
Cards on the table at one definite time.
How much the author had to sweat to get there, to the extent it may matter at all, should show up in the results--a ruling written with a measured, judicial temperament.