No, even when you do allow a site to run JS, NoScript includes additional XSS, XSRF, and "click-jacking" protections that aren't normally offered by Firefox.
noscript is per-source, so you can whitelist their <script> blocks and jquery.js, but that random javascript in an onmouseover in a forum comment will do nothing.
Maybe it's been a while, but I thought that NoScript was per-domain. In the event of a XSS, the javascript maybe included from the page's domain. NoScript wouldn't help you here. IIRC, NoScript wouldn't say, "Hey this script wasn't here the last time you visited this domain, do you want to allow it?"
The main gain is that you no longer have to worry about getting hit by this class of attack whilst browsing as normal. XSS attacks are happening all the time, even on major websites run by extremely clever techies. You think that is very little gain. To me, that gain is worth the hassle of having to manage NoScript. There is also a positive secondary benefit in that most websites which don't require js to work will run a little faster with NoScript enabled.
You might be right. I was personally hit by a Twitter XSS once. The only reason I enabled JavaScript on twitter.com was because you can't post (or at least couldn't) post new items without enabling it first.
I don't use the twitter.com website any more. Prefering to use clients that don't run JavaScript. Whenever I can use something other than a web browser to access a service, I will take that path. I use NoScript when that isn't an option.
I also found (and reported responsibly) an XSRF flaw in Linode.com a few months back that I believe has now been fixed. That was quite a dangerous one. I also found an XSS flaw in DuckDuckGo a few weeks back. Maybe this is the reason I'm so "paranoid" about JavaScript. Maybe I'm right to be.
Don't get me wrong, I can appreciate reducing your attack surface... but noscript just doesn't seem like that great of an idea, still.