There is a real difference between redshirts who do it as frosh (Albany, for example, but they are not alone) and someone who uses a medical redshirt after playing for 2 years or more. In one case, the extra year may not ever pan out, in the other case, the player is now a known commodity and the coaches can let him know just how he would figure in their plans in that 5th year.
Off the topic (a bit) but I believe our league's posture on non-medical redshirts kind of kills us in attracting quality (and otherwise qualifying) FBS scholarship players who are buried on their respective depth charts and are looking to move.
/\ The kid who is buried on the depth chart and plays in a handful of football games as a schollie FBS player might 'carry' a non-medical redshirt (or qualifications to successfully apply for a non-medical redshirt) to Richmond/Villanova/UNH but cannot do so at HC. (In other words if he spent two years attending Notre Dame or BC he could play three at Delaware but just two at Holy Cross. )
There has been some debate on this but I think that is the upshot of the non-medical redshirt impact at HC (perhaps not league-wide). This is not to be confused with hoops whereby transfers affected by NCAA rules requiring sitting out (not redshirting) is another story.