>> Are the score differences caused by the rewrite-to-BooleanQuery implementations ever "useful"?
So we need to consider what we are losing - TF, IDF, coordination, length norm, doc boosts.
I can only think of one use case which relates to coordination factor.
If you have a "category" field for a product e.g. given Lucene docs for these books:
Title: Lucene in Action
Title: The Long Tail
You might then use a wildcard search of /Books/Computing/* and "Lucene in Action" would rank higher than "The Long Tail" because a BooleanQuery would score a higher coordination factor suggesting LIA got more hits under this "/Books/Computing.." category. There would still be the issue of IDF potentially skewing results but the coordination factor is potentially useful here.
I think in general IDF tends to be useless for "auto-expanded" terms e.g. Wildcard, fuzzy etc. Incidentally, we still see that IDF issue in fuzzy queries ranking rare mis-spellings higher but that's another issue (one I resolved in contrib's FuzzyLikeThisQuery).
I suppose one other consideration is for people who have created any doc boosts e.g. trying to use this to boost by date.
I don't think any of these cases necessarily outweigh the benefit to be obtained from switching "wildcard/prefix to constant score queries"