Its been like this since wotlk at least. PvE players have to pvp for weapons to avoid pve rng crippling them severely. PvP players have to pve for trinkets & legendaries to rolfstomp.
I have no idea how much luck i have compared to avg player but i got 1 usable pve piece per week roughly in raids and my leg is from denatrius too. So i do benefit quite a lot from playing both. Wouldn't have gotten the ilvl without pvp to get into hc so quick and wouldn't have it so easy in pvp without dropping trinket & leg from denatrius. The benefits amplify each other. People who only play one section will lag behind in gear regardless of whether they play pve or pvp.