Browser plug-ins are a mixed blessing. They add useful functionality, but at the same time heighten your risk of viruses and other malicious code by increasing the browser attack surface. This is always a trade-off and sometimes the inconvenience of making sure the latest security updates are installed is worth the hassle. Google Analytics and Xero without Flash, for example, would loose their interactive graphs greatly reducing the functionality of both applications.
However, there’s a difference between a plug-in you’ve chosen to install, and one that just installs itself along with another piece of software. Worse still are those plug-ins that you just can’t uninstall. iTunes is particularly annoying in this regard, there’s no uninstall and deleting the plug-in files causes iTunes to reinstall itself (and the plug-in) on next run. Sure you can disable the plug-in in the browser, but I’d prefer it not to be there in the first place.