Whenever I see a post from someone whose username looks unfamiliar, I check their user profile. I haven't noticed anyone making a discussion post starting at single digit karma. In any case though, I doubt we would actually lose the contributions; if the users actually intend to hang around the site, they'll probably simply post them after receiving 5 karma; it's not as if it takes long to get that much if you have anything worthwhile to contribute.

Edit: actually, on reexamination, jhwendy did have single digit karma when he made his first (very worthwhile) discussion post, but his introduction comment alone was enough to account for 6 karma, so he should still have been able to make the post if the requirement were instituted.