Version 3.17 is not a required upgrade if your Movable Type installation is not affected by these issues.
Changes in this release include:
Installation at the root of a domain no longer continually logs out the user.
Fixed URL-encoding of uploaded file paths (e.g. images%2fbeach-pic.jpeg)
Fixed a bug in the nofollow plugin which causes HTML sanitization of everything within an <mtpings> block.
Fixed a bug in mt-db2sql.cgi which caused it to fail in certain situations when subcategories are in use before conversion.
Again, this update is not required unless you have been affected by any of the issues mentioned above.