In the pre-internet era, attribution to works as a license condition was a career and livelihood requirement. This persisted through the early period of the world wide web when it was extremely difficult to pinpoint original sources of data. In addition, the one-many communication model before things like blogs, youtube, forums and comments became both technically feasible and popular, meant that unless you owned or had access to a broadcast platform like a popular website or old media, then your voice would never be heard.
So where are we now? We're in a many-to-many communication environment where anything which is a good story will quickly bubble up. Example - the Obama "HOPE" poster - remember the kerfuffle about whether it was taken from an actual photograph or not and who owned the copyright? The story broke on blogspot
! This means that we're in a situation where someone else can profit from your work without attribution for a short period of time before it's discovered and becomes news, generating greater publicity for the original author. Even it it's just a fight between two internet forums, it's a simple matter to prove and document plagiarism and influence.
I bring this up because quite often I see people ask to reuse content, and are given the go-ahead with the restriction of attribution or non-modification. I'd like to argue the case that this is unnecessary and may even be detrimental to the spread of good ideas.
In a general case, is it better to have one person creating a derivative of your work with full attribution - or a dozen people working without attribution, but with the ability to gain the limelight should they strike gold? It's also a question of practicality - the effectiveness of a leaflet or poster may be diminished with attribution taking up valuable space.
Another upside - if someone creates a total piece of crap, without attributing you, then you can just sweep that under the rug and let it quietly die in obscurity.