Not my system. Just a happy user. Many things would make the system (any system) more predictive, including tracking weather, field conditions, rosters, even coaching strategies, and plenty of other variables that professional sports bettors (and betting systems) use as a matter of course. But I'm not sure weighting Showcases much differently than other games would be that significant. Of course everyone would agree the outcomes of the games are much less important - but they still provide reasonable data about how teams are performing. I'm not sure discounting them much would make much of a difference in using game outcomes to predict future game outcomes. It's the same with pre-season games. They mean nothing for the record, or anything else, but they are still a reasonably good indicator of how teams perform.
A lot of it comes down to what a rating should represent. Everyone agrees it should rank the "best" team first, and every subsequent team lower, all the way down. But defining "best" is up for much debate. What SR is rating is likelihood of winning a game vs. another rated team. The more games it can pick the correct winner, the higher its predictivity is. It optimizes everything, to make and keep predictivity as high as it can be (and it's pretty good at it). The team that is predicted to beat every other team, is by its definition the best team. But this isn't actually how many sports work. The team that wins the big tournament is the "best" team, even if a "better" team lost in the semis. The winner of the World Series or Super Bowl is the "best" team, even if they didn't have nearly the highest regular season record, and wouldn't be predicted to beat all of the other teams. In our world, the national champion of the highest league can legitimately call themselves the best team, but they may or may not be ranked #1 in SR (or even top 10).