I don't avoid JavaScript because of security concerns or because I personally run an older browser. I avoid it because JavaScript is a suck ass language. I have coded some reasonably large chunks of it for different jobs, and what I learned is that
- every browser does it just a little bit differently
- new browser releases will often break your code
- browser makers will not tell you all the details of how their version differs
- you will have to test it on every OS/browser combo you intend to support
- even if you get everything right, your pages will sometimes fail for certain users with no apparent reason
I have never wasted so much time debugging anything as I have with JavaScript, and it's maddening because the fault lies in different browser implementations rather than in your code. Combine that with the horrible things most people use JavaScript for and it's hard to see what's so good about it.
One final note about the browser upgrades: commercial sites have to support the browsers that their users want to use. You can't just tell your potential customers that they must download the latest gigantic browser release for the privilege of shopping in your on-line store. Yahoo generally does a nice job of walking the fine line and using JavaScript only where it is useful. Even so, their pages sometimes fail for me in the latest browsers because of JavaScript problems.