My opinion.....if they suck and you can do better, then eliminate them. If they are mediocre...then you might not be able to get better recruits and would be in worse shape.
The key: you have to be able to recruit. Cutting players and then taking even more sim recruits is a death spriral. I know this from my first four seasons playing this game.....
BTW: not all sim recruits are total crap. I had a sim DL recruit that became a 3 year starter (well, my team was not very good which was in his favor), and he ended up having the 10th most sacks in a season (at that time...which has since been bettered by others).