According to other posts here, being rusty adds a "random" factor. I would presume that a better player will, on average, play better than a lesser player if both are rusty, but since every pitch/AB is an individual event, that doesn't mean much....

There isn't a lot of information on the effect of "rust" in either the manual or here in the posts. There is no information as to whether more rest=more "rust"... There are a couple claims that the "rustiness" wears off during a game, but there has never been any confirmation/denial from PB.

I suppose is they answered that question, it might not be so "random".

I don't know if they should turn it off, necessarily, for the playoffs. I guess it's a question of how well the program deals with it. A better solution would be some way to increase control of the roster during away games.
For now, the only other choice is with 2 outs in the 9th, defensively substitute your entire bench, and have each of your pitchers intentionally walk one batter each.
Of course, if you're in a 1-run game that you have to have.....

Invest in Rust-o-leum.....