For one, this is on-demand and doesn't rewrite the default keybindings of the browser. It is nice for anyone else using your browser where they can ignore it entirely. It's a tradeoff and each has their benefits.
I can't live without vimperator nowadays. It's the only thing stopping me from switching to Chrome from FF (and yes, i know there are other similar vi extensions for Chrome but none seem as good as vimperator for FF).
Does Vimperator allow for vi mode in textareas? I thought it was for general browser control.
Just tested this and Vimperator does allow vi mode in text areas if control+t is pressed. It's not as slick as wasavi but it works. There's also the option of launching an external gvim window when control+i is pressed inside a textarea, which is a cool feature for more in depth editing tasks.