It's a matter of overall impact. Most good teams have good WR's, and those good WR's are typically the most dangerous offensive players. Guys like Gronk, Kelce, and Kittle are the exceptions. But, the point is that Smith isn't covering those players, and he isn't impacting plays that go to those players unless he gets home on a blitz. He's a very good blitzer, but he's not D. White in that regard, and his production as a blitzer doesn't compare to the production (impact on the QB from pressures and sacks) of a reasonably comparable Edge.
So, Off-LB's are less valuable in coverage than CB's and S's, and they affect the QB much less than Edge - or a disruptive DT for that matter.
And, that's just defense. You're not even in the conversation without high-end weapons. KC, GB, LAR have competed with relatively poor Off-LB's. It's not that they don't matter. Those teams would have been even better with better LB's, but it clearly doesn't affect the bottom line (W's and L's) the way WR, Edge, OT, CB, and S do. DT and IOL also have strong cases over LB.
