I thought the manual said that rustiness introduces a "random" effect into the pitchers performance. This is not necessarily always bad. It could be good as well (at least that is what I have heard in the past)

I do have a tendancy to use rusty pitchers in relief or more often I use them in relief to keep them from getting rusty. But one word of caution-- I once lost a playoff game in a Launch League because I had a 3 run lead and I wanted to get all my players in to keep them from getting rusty. I ended up blowing the lead and eventually the series.