Say someone types out something very offensive or insults another members -- if they delete the post before staff can see it, the evidence is gone.
Exactly.
Members are still able to modify their posts anyway.
I think you're missing the point.
If the staff can't get there fast enough, it means they DIDN'T see it.
If they don't get there in time, they can't get a screen shot.
So your idea is kind of flawed.. a lot.
With this new fix, where members can not delete their own post, we can gather screen shot evidence while still keeping the actual physical evidence there, so that the member can't claim that a 'staff member made the image up'. (Someone out there is likely to true and pull that on us, give it time.)
Besides, like Raksha said, you can still modify your post. If you want a thread of yours deleted, you can always PM a moderator. If you make a post and decide you don't want to say whatever you said (assuming it is not offensive), then just edit the post.
I really don't think this is up for negotiation, guys. xD