More specifically, if you include S as a possible letter, it appears in the second-most words (53,556, compared to 63,445 for E). But once you have some words it's natural to just stick S on the end of everything (this will work for most nouns and verbs) and this makes the standard spelling bee puzzle uninteresting. (I don't think it makes the meta-puzzle less interesting, though.)
If you run Robinson's code but omit the filtering out of words with S, the best puzzle is E/AINRST (8681 points). The previous winner, R/AEGINT at 3898 points, is now only 442nd-best. (Notation: I put the required center letter before a slash.)
Interestingly, one of the five puzzles that's tied for 15 points, the second-worst possible, includes an S. The puzzle is Q/CIORSU - the words are CROQUIS and SUQS.
I don't understand. Why S?