My motive, at least right now, is to figure out which players are adding win probability, as a method of evaluating performance. To me, it’s just another piece of the puzzle for player analysis. As a result, I prefer not to include all of those complicating factors.

At the same time, it would be interesting to see it implemented with all kinds of additional adjustments (home field, batting order, individual player performance, etc.) to get the “real” win probability.

Ultimately it all depends on what you’re intending to do with the win probability calculation. If you’re trying to accurately assess the probability of Team A winning the ballgame, either for entertainment or gambling purposes, then of course you make use of as much information as possible.

If you’re trying to use win probability as a tool to either evaluate past performance or to predict future performance, and you’re intending to compare players who hit in different spots in the batting order, then I’m not sure it makes as much sense.

For example, if you changed win probability figures to include batting order spot, then any decent NL hitter’s WPA would go way up if his manager (absurdly) started batting him 9th.