I am currently in my 17th season of my league (154 games a season), and our league is centralized similar to the one described earlier in the string. We simulate games in 6-game batches. For each of those 17 seasons I have kept a spreadsheet on which I plan out my starters for the next batch of games in one column, and then after the results are posted I record the actual starter (along with pitch count) in the next column, and then I use that information to plan out my rotation for the following batch of games. I have never, in 17 seasons, inserted anybody in the Spot Starter chart. I've had plenty of injuries in those seasons and usually use the Strict setting and probably use 4-man about 75% of the time (we're in the 1920's-1940's), but I have never seen the AI skip to the next pitcher in the rotation to cover a hole created by an injury. If, say, the # 2 starter is injured and his turn comes up again, the AI has never just skipped to the # 3 starter--it has always plucked an appropriate spot starter from the bullpen and used him instead.

As for the Spot Starter function, we've had some owners who tried to use it a lot but had trouble figuring out how it worked, so I can't help you on that question.