Wouldn't that lose all the cached query plans every deploy? Could be wrong, bit rusty on that stuff now, been a while since I worked on something where we had to worry about that.
Then when new version is being deployed, the tool compares the database with the XML, and generates SQL to alter the database.
For stored procs etc it compares the text of them, so those that weren't changed are ignored.
It's a simple homebrew tool but it gets the job done.
what you probably wanted to mention is table statistics - but they are cleared only if you truncate your table, but then again - once you populate your table - the engine will recalc statistics by itself.
overall RDBMS does a lot of stuff behind the scenes for you, and you should take advantage of it, and instead think about more important atuff - business logic, data modeling, schema evolution, etc