I might be totally misunderstanding the concept of web-of-trust, but imagine the following scenario: I generate my key, then go to a key signing party, and after, I import all the keys which fingerprint I have verified, and sign those. Now, this will make all those keys fully valid, but the default trust for each key will still be set to the default, i.e. "unknown". Which means that if I now import a new key, even if this new key has enough (*) signatures from those, it still won't be considered valid, because none of those keys is trusted.
Which means that for key signing parties to have some usefulness, we must set those keys' trust to at least marginally trusted. Right? Or am I making some mistake somewhere in my reasoning?
(*) - In GPG's default security model, i.e. one sig from a fully trusted key, or 3 from marginally trusted keys.