I strongly think it is and obv. this lowers the overall quality of this forum as a source for useful information. As soon as a newbie asks a question, sig spammers start crowding the topic with their 99% sensless/false/bs answers. Some (most?) of them didn't even understand the topic!
@Theymos: The problem is not created by the sig spammers, but the system allowing it. There might be a point where Bitcointalk will be rejected by the users, when you can't find the information you are looking for, because it's hidden in a flood of trash.
In my opinion, there are truly lot of posters who posts senseless replies on topics here in bitcointalk. In order to stop this, i think we need to have a better rules on Signature campaigns, accepting users in campaigns ,scanning applicant's posts thoroughly would make at least prevent spammers, but this cannot be easily done. I also suggest that we should have a thread for where we can posts evidences and proofs who have been spamming and posting senseless things so that we mod can temporarily ban them or give them warnings. Again, this is only my suggestion, i know that this is cannot be done easily.