Yes. IMO having people participate in a discussion before starting one is important. It forces the poster to get a general sense of the site and its (un)spoken rules before starting a discussion.
This is one of the foundations of building an actual sense of community. If you were having a discussiion in the real world, wouldn't you feel a bit put-off if someone jumped in and started talking without spending even a moment to hear what other people are saying?
Requiring that someone has SOME positive karma before posting is just a way of saying "yes, this person has shown some interest in participating in a reasonable manner".
