Yeah, the advantage from legendaries is there, but they are crazy expensive.
Running 3 legendaries and to farm the shards you need in-match is something only very very few players do - because its not as badass as it might look on paper.
Here downsides of triple legendary loadouts and why most players don´t o it:
You starve you team by gathering all shards for yourself. Buildables/elite minions might not be built in time to push/prevent enemy push.
You´ll need precious time for shardhunting, while you should hold the line and destroy minions with yor team. This can enable enemy pushes.
you´ll have your gear running extremely late. In opposite a simple grey gear part can give you a big advantage in early game.
with the new changes on the Incursion and Meltdown mode time in lane is even more important, same for buildables and elite-minions. To run triple-legedary gear is simply not wise, and until now most know that.
PVE is not so hard. I almost exclusively play PvE-missions, only on normal difficulty, and I´ve legendaries for days. And I´m not even a grindy player.
PvE is also ment for new players to gather skill & gear.
Not saying its a kiddie-pool, but I think its important to get comfy with a character in PvE first, to know the mechanics, movement, skills.
While you play 4-5 missions you learn the moves and you pick up some hopefully usefull gear. Thats 2-3 hours only to gather alot experience & gear for a good start in PvP.
Plus: You can get legendaries from lootpacks. You can get lootpacks by playing PvP through credits/ Command Packs. Legendaries are not PvE-exclusive, even if it might seem so.
I could go on rambling about the importance and glory of PvE, but I´ll spare you with it
PS: awake for 23 hours, I apologize for typos >.<