I don't even need to make a case. Everyone knows this is a true statement.
[Editor note, from OP comment]
For the sake of making a case let's look at Arrow Season 1 and Season 2.
During season 1 there was a holy crap moment when we all found out Tommy's Dad was the black archer and was trying to destroy half the city. Then Tommy died at the end of the season. There was a cohesive plot to the entire season including the flashbacks.
During season 2 there was a build up to the big reveal that holy crap Slade Wilson was alive and ready to deliver a beating. The whole season built up to that point at the mid-season and thrived from that point on.
During season 3 there has been zero build up to the mid-season finale in which Oliver had a fight with Ra's who now wants to be his friend. Holy crap Arrow you've lost the plot.
The Flash has Grodd and the Reverse Flash and has been steadily building to an epic end to the season. Just watch the trailer for the end of this season of the Flash. It makes you want everything right now. If they made one for Arrow you would be like EH.