Valve has taken the time to discuss why exactly they make their protagonists silent, this time specifically talking about Chell from Portal and Portal 2.
"[Chell] could be anyone," said Erik Wolpaw, a writer on Portal 2. "It could be you. There's the silent protagonist, which I don't know is specifically a Valve thing, but it's a thing. It's probably a little bit tougher in Half-Life, because it's slightly more realistic and there's conversations that are happening and you kind of have to write around why Gordon isn't saying anything."
He then went on to say that if Chell was to speak, it would probably take away from the other voice performances and make them seem less important.
"You may want to know Chell's backstory, you may want to hear her say things, but I guarantee, if she had to say her straight man lines at the expense of half of the other dialog, it would suck."
Source: MTV Multiplayer Blog