Just because raid weapons are legendary doesn't mean they should be out-performed by exotics. The raid is the hardest end-game content that Destiny has to offer, and the raid weapons should definitely be amongst the best rewards in the game. VoG has shown us that it's legendary arsenal is pretty damn impressive (Fatebringer, Praedyths, VoC), and they have elemental damage attributes to boot.
Exotics should be great, but really shine over other weapons in certain niches/situations. They should be able to augment or change a playstyle in ways that legendary weapons may not be able to. That being said, many current exotics obviously fail to live up to that. I think Hawkian has shown what works well in the OT with his build randomizer. Judging by the upcoming changes to exotics, we are about to see even more crazy perks and build potential.
When we look at primary exotics that have elemental damage types, we only see one (Vex Mythoclast). I know a lot of people feel that primary exotics should have elemental damage attributes, but I'm still not sure if it's even necessary. Giving the raid weapons elemental damage types helps make them more viable in situations where you want to use an exotic special/heavy, and encourages you to use exotics in other slots. For a special gun like the Mythoclast, I think it's deserving of the solar burn.
Destiny has an arsenal that promotes frequent change. You can really switch your build up for different situations and encounters to get the very most out of every situation, with the gear you have.