There's plenty of reason to have multiple browsers at any given time. For one, it gives you perspective on which caters most to your system / preferences. You like the lightweight feeling of Chrome? Great! Use it when you want to view Youtube content and have no need for extraneous features. Enjoy Firefox' clinical approach to open source development and modification? Fantastic! Fiddle your little heart out with its about:config material. The security implications are far outweighed if the user is clever about their surfing habits and clears their registry more than once a week.
My all time favorites after extensive testing are Chromium, Chrome Canary, Comodo Dragon, Comodo IceDragon, and Firefox Nightly. To list the benefits:
Chromium is stunning with its release schedule. A BuildBot console overview gives you plenty of oversight on fixes and their stability. The browser itself is rich with simplicity, cutting edge technology, and is devoid of the Google goon privacy statements that push many away from the standard Chrome builds. Open source developmentis much appreciated, as is the stripping of bloatware that your web browsing experience shouldn't require in the first place.
Canary is a step down from the aforementioned. It fails to respect the user's privacy in some ways, but still caries a few of the features that certain websites need to run correctly. It is almost as fast as straight up Chromium, too.
Dragon is amazing for privacy, on the other hand. No other web browser has supported the stability and speed of this Chromium build, nor do they come close to the codec-support it offers. As a platform, it is severely underrated, and cuts the balls off its main competitor, SRWare Iron.
IceDragon is the same program as its WebKit counterpart, except for the fact that it is based off of the Firefox-famed Gecko layout engine.
Nightly is the Chrome Canary of the Firefox community. It provides innovative technology that few other services have yet to do. Compared to Opera Next, it is a lot more stable, and even surpasses vanilla Chrome in speed and security tests on several websites.