The Google Talk numbers I've seen published elsewhere are that ~5-10% of sessions run over TCP, relayed through a server, because UDP doesn't get there.

[BA] On Live Meeting, it is quite frequent for at least one user in a conference to require a non-UDP alternative. Where UDP outbound is not allowed, we are finding that often TCP outbound is not allowed either, and only HTTP (and sometimes not even HTTPS) will work.

The reasons to prefer point-to-point UDP if possible include:
- Much lower delay when the endpoints are close to each other, network-wise
- Much cheaper provisioning for the service provider

No argument. It doesn't make sense to prefer TCP or HTTP transport if UDP is available.