Idk that you could call it misused. When 4-3 teams take undersized rushers they tend to move them to Sam LB where they are matched up with guys more their size at TE in the run game. Then on 3rd downs they switch to Edge Rushers. The entire idea is to protect the undersized player from having to go against Olineman in the running game on a down to down basis.
It's how you get most value ie a 3 down player, otherwise he just becomes a situational rusher.
The greats are capable of doing it, lets not forget in Von Millers 1st year with Broncos DA was coordinator and Miller played Sam effectively and rushed the passer on 3rd downs. As did Khalil Mack. Both were under 260lbs coming into the league.
Baun and Kikaha were under 250
Still, in todays pass happy NFL, even if the guy is only a pass-rusher having a designated one still boost your defense.. You just have to protect him in the run game, so in that regards I agree...Saints hadn't smartened up about using primary passrushers. They are still focused on guys that can do both and that's why they draft 6'5 270 lb guys