It can depend greatly on what the cam model's main site is:
If they're on a private based site like SM often models will run their skype ads at the same time like skyprivate, Cammodeldirectory, niteflirt etc so they can easily respond between private shows on SM. I'll stay logged into my listings an hour after or so I'm done with SM if I feel up to it.
For a place like MFC it's important to stay consistant but it wouldn't hurt either if the model was that interested in skype to make listings on indie sites to log on after they stopped camming on MFC for the day. If it's super slow on MFC sometimes it's better to log off than kill your camscore hoping for new people so it would make sense for models to supplement their income with skype if the night is completely dead.
Is it worth it? That's totally individual. I've had my skype earnings help my main site income when it was slow. But I've also heard other models not liking it for a variety of reasons. (Dealing with timewasters,not liking the super personal aspect etc)