This is something I've been thinking about some time, and I might be completely wrong....
Is it necessary to take fish oil???
The Paleodiet book is talking a lot about the balance between Omega-6 and Omega-3. And if you read studies about Omega-3 on pubmed, the positive effects are reached by improved ratio between O-6 and O-3.
Are there any ailments that are improved/cured directly by taking supplemental O-3? Not by improving the ratio between 0-6 and O-3.
If the only thing Omega-3 does is improving the ratio between Omega-6 and Omega-3, then making sure that you are eating a diet with the right ratio between the two would be a way to not have to spend a lot of money on supplemental Omega-3.
And as I said in the beginning, this is just speculation from my side. I'm sure there are some experts here on the forum that could shed some light on this.