We're at a point where we don't even know what we don't know about nutrition, so it's fair to have "beliefs" like this I suppose. But there is substantial evidence for DHA's impact on brain development if nothing else, including the brain development of children birthed from mothers low in DHA stores. Not much significant evidence suggests equivalent impact from ALA/plant-based fatty acids, nor do they convert predictably in the body to forms the forms that are actually directly useful. I will grant you that EPA is more debatable, but DHA is pretty solid. And while I don't trust speculative 'evolutionary' evidence, it does make sense that human brain development (and thus human development in general) supposedly started to take off once they migrated near the coasts where there was an abundance of seafood to hunt.

Not to mention that plant fats/oils are the largest contributors to the problem of having imbalanced omega-6 to omega-3 ratios to begin with:

http://nutritiondata.self.com/foods-000141000000000000000-w....

Either way, "essential nutrients" and nutrition in general aren't things that we can assume as being settled, because much of what we "know" about all that will almost certainly become outdated in a few short generations. In the meantime, I think moderate diversification of nutrients isn't a bad idea.

Edit: I also made no mention of vegetarianism/veganism, I simply referred to plant-based food sources specifically, because many animal-derived products (including eggs as you mentioned), most certainly still have the benefits I was describing. It's not about lifestyle or ideology, it's about finding quality sources of things that promote optimal human functioning.