Connecting to 20 idle channels results in a steady flow of 100 KB/s of traffic both ways. A bit too much for doing absolutely nothing, I suppose.

As far as I know, it does not do any anonymizing message mixing or traffic pattern observations evasion (users are directly identified anyway).

And that's for channels having just dozens of users. Maybe the protocol is specifically designed to handle 10000 users per channel without increase in bandwidth, that's a different story, and I can understand that some initial bandwidth quota can be needed for that. However, it's not mentioned anywhere, and I suppose that's not the case.