EDIT: I am using FCE 3.5 HD on Snow Leopard. Is that bad?
My general opinion it's never bad to keep your money in your pocket and keep using software that works.
The last version of final cut express is the same as the last final cut pro (before X was released on the app store) so it is as capable as pro for multiple camera editing.
The oldest Express version I have handy is 4.0.1 and it doesn't need any plug-ins to work for multi cam.