I'm not the person you asked, but here's my reasons.
I'm on Linux and there no longer is a supported version of Flash by Adobe. (Apparently there is some NPAPI to Pepper bridge though, to get Chrome's Flash working in other browsers.)
I already uninstalled before that, because I wanted to force HTML5 for sites that dynamically switch between Flash and HTML5 and because I wanted to nudge more sites to support HTML5 by boosting the stats of people who don't have Flash installed.
> I'm on Linux and there no longer is a supported version of Flash by Adobe
I'm using Firefox on Ubuntu with Flash (flashplugin-installer) and it's still receiving updates. It's an older version, but it has been working fine with every Flash site I've visited so far.
I think everyone got most of my reasons covered below, but here they are: no need to manage updates, smaller security surface, battery life, fan noise. I typically browse with Firefox Nightly, and without flash installed globally I get HTML5 or whatever a site wants to offer me if I don't advertise flash compatibility. If I hit some must-have flash I have a key combo that opens the current tab in Chrome.
Historically, it's tended to have had far more zero day exploits than the web browsers themselves. removing Flash removes a huge attack surface, and 95% of the time you don't need it.
I don't have flash installed (usually using Safari and Firefox) and have to jump over to Chrome to use a site maybe once or twice a week, which is more than acceptable given the insecurity of Flash.
I really don't like flash. It's full of security holes and its primary uses annoy the living hell out of me. So I only use a browser that has it when it's on my terms.
NOOOOO this is what Chrome is for! The only flash I have is the built in to chrome flash.