I think by enabling any scripting language, you create a potential security risk.
But the main thing is, I think, is that JS has a lot of things in it that simply aren't going to work on a blackberry, like launching multiple browser windows, or doing mouse-over actions.
Does anyone have an example of a site that uses JS that won't work on a blackberry?
Prior: BES: 7510, 7520, 7290, 7230, 7130, 8220 (No Data Plan), Droid Eris
Post your question on the forum, don't PM me.