It's also just one more stupid thing running in the background. Also, there are occasionally patches that cause problems with something else but get downloaded anyway. I prefer to check out the update site myself every now and then and know exactly what I'm getting.
By the way, what's with the Firefox pimping banners in some people's forum images or signatures? I mean, if you know what the banner means and why FF is so great, you probably already have it (or Opera or whatever else; something besides IE), and if you don't the link is somewhat confusing on what exactly FF is.