I'm not sure how including a final semicolon can protect against an injection attack. In fact, the "Bobby Tables" attack specifically adds in a semicolon, to be able to start a new command. If inputs are sanitized, or much better, passed as parameters rather than string concatenated, you should be fine - nothing can be injected, regardless of the semicolon. If you concatenate untrusted strings straight into your query, an injection can be crafted to take advantage, with or without a semicolon.

Wouldn't that still apply, if you can inject straight SQL, such as "query' OR 1=1?"