While you do not get all the benefits of a properly prepared (and repeatedly re-executed) SQL statement, using placeholders still is a significant improvement even when using do, because the SQL is typically also cached in the DB server.

So when you do the same SQL string the second time, it does not have to be reparsed from scratch (server-side). This even works when someone else connected to the same DB uses the same SQL, which is a good thing because you are probably using more than one connection at the same time.

Conversely, not using bind variables can totally kill the scalability of a database application.

Of course, re-using prepared statements where possible is the best way, but using bind variables is itself a great (I daresay necessary) improvement (and a good habit, for both performance and security reasons).