Since when do coaching staffs guess when evaluating players ? They look at film ; the put players through various drills to guage what they can and can't do.
Just because a DE drops into coverage every once and a while on a zone blitz, doesn't mean that they can make the switch to OLB in a 3-4 scheme. When DEs do that in a 4-3 scheme, the point is to suprise the QB; for the most part, if a player has his hand on the ground, he's going to rush the QB. Some zone blitzes have DTs dropping; surely, it isn't ideal to have a 300+ players dropping into coverage on a regular basis.