In my experience it is true long term, because disposables have the "stay dry" effect and cloth nappies don't, so it's best to change the cloth nappy as soon as possible when it's wet.

I once bought a pack of disposable boosters for overnight use, but once I'd used one I couldn't really see the point — might as well just put a disposable nappy on. I haven't tried cloth boosters but it seems to me they'd have the same wet-against-skin problem as the nappy itself. I suppose people do use them successfully though.