If you are looking at the signature campaign as a source of income then it is for some people certainly a waste of time.Especially in the case of low-paid campaigns.On the other hand if you participate in the forum to learn something and help others, but at the same time you have the opportunity to to earn something, that is certainly not a waste of time.
Its really depending on the fact what kind of forum rank you are having and also how much spare time and sign you are having.
If you have a well payed sig campaign its no problem and worth it.