I'm a web designer/developer and have used Flash Optimizer many times to reduce the file size of SWF's to get them where I need to be. Sometimes i only have bitmap images to work with or have complex vector graphics to build animations for banner ads. I can usually get the size down utilizing other tricks. But, then sometimes, I need to squeak out another 2, 10 or even 20K out of a file size to get it to meet banner requirements for placement. Sometimes that small banner size limit is harsh! Flash optimizer does the trick. While there is usually some play to manage how the file is reduced, you can tweak many different aspects to manage what part of the SWF is affected. While it's not something everybody will use, if you work in an agency or on your own to produce SWF banner ads that have to meet a tiny file size requirement to be placed, Flash Optimizer will do the trick.
The biggest con about the ap is it may take a bit of practice to understand what settings to tweak to get the file to reduce the parts you don't mind and maintain the parts you do. Even then, it has come in quite handy to have in my toolbox of applications.