If I'm reading this correctly, your suggestion would alter a single table online, and at the end, I would end up with a table with a new schema (assuming I had no foreign keys referencing the table being modified, which seems to introduce additional complications). Presumably, this change happens while my application was running, which means that during the migration, I would have to use the old table format, and then cut over to the new one instantly once the migration has completed.
Our migration at the time involved multiple table changes, many of which had foreign keys referencing each other. It doesn't sound like this tool would atomically switch all tables to the new schemas, which would have led to broken data for us. Does that make sense?
EDIT: grammar