One thing to note - since the db engine is running in the same process as Excel it can interact with it. There are some nice things it can do because of this, for instance, SQL-based conditional formatting (e.g. there's a SetFontColor function), calling VBA functions from SQL and a bunch more.
There are also some other quite cool functions, e.g. replacex and regex_split, levdist, eval and others. The QuickQueries dialog has some built-in queries to check out if you're interested in what these can do.
As for permanent licenses, I understand the concern. If I do end up going offering permanent licenses for professional licenses, I'll either upgrade the existing licenses or offer a paid upgrade to the permanent license to cover any price difference.