I think DELETE is still very important for backward compatibility. There are many cases when removal of user accounts is more convenient through DELETE than it is through DROP USER – for example, when a set of accounts with certain characteristics needs to be removed, or when the server runs with skip-grant-tables, e.g. in "repair" mode; and while the right way nowadays to do it would be removing them from global_priv (and other privilege tables), certainly there are still lots of scripts out there in the world which do it through the old mysql.user.
UPDATE, on the other hand, has very limited functionality when it's done through the view. So, while some users may use it still, losing it shouldn't be that critical.
I think DELETE is still very important for backward compatibility. There are many cases when removal of user accounts is more convenient through DELETE than it is through DROP USER – for example, when a set of accounts with certain characteristics needs to be removed, or when the server runs with skip-grant-tables, e.g. in "repair" mode; and while the right way nowadays to do it would be removing them from global_priv (and other privilege tables), certainly there are still lots of scripts out there in the world which do it through the old mysql.user.
UPDATE, on the other hand, has very limited functionality when it's done through the view. So, while some users may use it still, losing it shouldn't be that critical.