Sources become outdated when they cannot be contacted via the internet. This can happen if the computer the other client runs on is turned off or has switched its IP and we can do nothing about any of those two.
With the webcaches you are right. I dreamed about it last night and it is bullsh*t. But why are outdated sources stay in the queue for weeks ? Can't they be removed after 50 reconnects without getting them, or maybe 7 days ? Automatic search is quite a joke, when having so much outdated sources, never removed.
Well, there is always the chance of a source being reconnectable after even a larger amount of time. The problem is that this is a question discussed over and over on these boards: are longer or smaller timeouts better? Truth is, there is no right answer to that question. However you can set the amount of reconnects Shareaza will try before dropping a source within the advanced settings.