I suppose it's a matter of preference: if you feel happy letting any website you go to run any script it likes on your PC, then don't bother. If, however, you value security Noscript is very useful.
One of the greatest browser threats are exploits in the Javascript engine. By default, Javascript is limited it what can do. Among things it can't is the interaction with files on the host outside the browser directory. It's a sandbox, if you will. However, bend the rules and you get out of the box. Theoretically, there could be an unpatched vulnerability in the browser Javascript engine that might allow escalation of privilege and execution of code outside the browser sandbow. Well, this happens all the time. Vulnerabilities are found and patched, not only in Firefox, but in all browsers. However, since Firefox has a built-in autoupdater and a very fast patch cycle, the chance of a user stumbling across a malicious script capable of exploiting the yet unpatched hole in the browser is quite low.
Apart from security, quite a few websites are too noisy for my opinion, with too many elements dancing, drawing your attention and distracting you. Call me old-fashioned, but I just need the raw, naked content and little else. Most of the time, I merely wish to sit down and read what's there. Noscript makes this task easy as it filters away some of the potential garbage, making sites load faster and with much less background noise than they usually do. Of course, there are sites where you require the functionality. This is exactly what the whitelisting means. Enable all and everything on sites you trust - but also on sites you want or need. Very simple. It does require discipline and is definitely not meant for everyone.
