Exotic equipment is perfectly fine for Open World stuff, Raids, Strike Missions and low level Fractals. The difference in stats between Exotic and Ascended is something around 5%.
Ascended equipment is required for T2+ Fractals because you need Agony Resistance (infusions).
Legendary gear is literally the same amount of stats as Ascended, but with free skin and stat change — it's mostly QoL improvement.