The problem is taking players and using them in roles that are not their bread and butter. Personally I could never understand this "lets take a 4-3 DT and make him a 3-4 DE" or "lets take a 4-3 DE and make him a 3-4 OLB". The positions are different and have different responsibilities. Taking guys out of what they do best and expecting them to be great at something that they've never done is not fair.
Some guys can do it, but I'm willing to bet the vast majority never live up to the expectations.