Regarding your addendum, I too have joined a channel to find everyone idling. Turns out, they get an alert when someone writes something, and all you have to do is ask your question. Almost invariably someone will pipe up.
They'll pipe up if the question is something like "What's the flag to make ls display in color?". IRCers are generally pretty quiet if you ask something actually technical or involving the code, or they'll just offer up a lot of speculation and irrelevant links. My theory is that the real project leaders and developers use mailing lists because they need to get work done, not watch IRC.
Definitely true. It can be really great if the main developers are actually using the IRC channel to coordinate development, but I think that can only really work when there aren't a ton of users to be poking in all the time. Kind of a Catch-22 problem; Linus would not be well-served to hang out on a Linux channel, because (as anyone who has seen his Google+ posts can testify) an army of sycophants would feel the need to interject their own non-content responses to everything he says. A small project, however, can still coordinate among developers while helping the odd user who shows up.
That's exactly what I did (I was aware enough of IRC ettiquite that people idle in the rooms but may come back sometime later & answer): ask a question and usually answer my own question and quit without a response some 4 hours later.
Which is better depends entirely on what the question is. Most questions asked on IRC have absolutely no business on a mailing list, and the ensuing discussions would render the mailing list unusable.
Nobody advocates abandoning email for IRC. Right tool for the job, like anything else in life.
Think of them as being "on call."