To be fair, those have been very effective in keeping my non-techie family members from accidentally installing malware because they'll at least check with me when presented with a scary system dialog. And it hasn't impacted my own use other than the occasional extra clicks. In a locked-down corporate setting, I can see how that could be maddening, though.

That's for the benefit of people who don't really realize that installing software from the internet is dangerous (i.e. most users). This discussion is about safe methods once you have decided to install something from the internet, which is different.