They should go. Especially if they're a RB, WR, LB, or DB.
Between the current rookie wage scale and the many and great risks inherent to football, I think it's foolish to return and turn down $1M+ / year and a guarantee of at least half that (the guarantee percentage gets pretty small after the 2nd round). There's just too much that can go wrong in the interim.
Get in the league. Get to your 2nd contract a year earlier. And then get out with as much of your health as you can (and come back to get your degree during offseasons)