So basically, what you get is your comment, with a link to the post you're commenting to.
At most I could see this as useful if you're trying to rec someone's fic or post, which I generally wouldn't do on facebook to begin with. Not because I hide any connection between my facebook and my livejournal, cause I don't. But because the people I've got on facebook aren't really interested in fanfic. (and believe me, I've tried to get my mom or brothers and sister to read mine*g*)
Aside of that I don't get why people are so against the ticky box, I tend to be of the opinion that it might be a bit useless, but other than that, all you have to do not to use it, is not to tick it on.
I will tell you though that if you do click it on, you don't get a delay to stop it from posting, not even a reminder of 'this comment was crossposted to facebook' or anything like that.
So take that as you want.
PS: no worries, I'd never crosspost any comments I make to a locked post or to a fandom post to begin with. If I wanted to link to a post, for something like gay marriage or stuff that needs to get spread, I'll ask permission first. That ok?