Omega3 is linked to all kinds of long term health outcomes. The evidence for it now basically makes it retarded to NOT take a fish oil supplementBecause people are walking around sick for lack of omega 3s? When the answer to the questions "what will happen if I take this" and "what will happen if I don't take this" are both "nothing" then just save your money.
As for other vitamins, my policy is that vitamins are so cheap they might as well be free, and I don't always eat perfectly plus i exercise a lot, so why not. The possible gains are massively larger than costs