As a farm administrator, you will definitely need to restrict SharePoint Designer usage in many cases.
For example , you might need to totally prevent users from connecting to SharePoint sites using SharePoint Designer 2010, you might also need to disallow pages unghosting and in many cases you will need to prevents end users from managing the web site URL structure.
Fortunately, in SharePoint 2010, farm administrators can manage SharePoint Designer settings at the Web Application level from the central administration as shown beneath. This way, the farm administrator will restrict all site collection administrators and revoke some privileges from them.
Also, there is a similar screen at the site collection level, from which site collection administrators can restrict other users.
For example as a farm administrator, I have disabled Managing the URL structure at the web application level. Now, when site collection administrators check the SharePoint Designer settings at the site collection level, they will get the screen below.
Hope this helps 🙂