A channel has a lot of members at all times. Someone (new to rp) comes in and does some things. Tumbleweeds or spontaneous rp occurs.
Later that person is in channel, lurking, and some of the previous lurkers act out a pre-arranged scene.
sometimes, it creates the impressions of "chatlog farming", or of there being a definite "In-crowd", or that you're interrupting something, which can be intimidatory or depressing, depending on how things were at the time.
E.g. If someone goes to a place, and the first time, it's tumbleweeds, the second time there's more tumbleweeds, but a lurker posts a dreaded bracketed comment inquiring about spelling or something (thereby indicating they are paying attention but don't want to rp at the time), and the third time, theres a scene involving some of the lurkers that were there the first two times, then it does not tend to form a good impression of a place.
So... lurking can be a bad thing.
On the other hand, a channel about a topic, e.g. something about sciencey things, then lurkers are not a bad thing, as a sufficiently populated channel suggests a lot of people having an interest in that topic.