...unless you want to implement some sort of server-side automated correcting that improves bad quality comments for instance. Rare, and possibly not very practical, but I've seen attempts at it here and there.
But I want to add domain-specific wordlists to the spell checking in my web app. I don't think this is that unusual a request.
More bloated software, yeeeey!!
I'll take the software bloat over the network bloat from the extra requests sending every keystroke over the wire.
On OSX at least spell checking is a function of the OS rather than the browser.