I agree with you that Tolbert is not a bust by any means. He's a solid NFL receiver, definitely not a 1 or 2, debatably a 3 or 4.
Some could argue he's never been given the opportunity to excel, others could say last year he got a lot of run and ended up with statistically pretty solid numbers (49 catches, 610 yds, 7 TDs). Those are solid #3 numbers.
I think the Cowboys hoped he would develop into a #2 (as a 3rd round pick), but he did not. He also got passed as the #3 by Turpin. Now he's out of time as his rookie deal expires. For the Cowboys, it makes sense to try to get something for him, as he won't be brought back on a 2nd contract (because he's not a top 3 here), and we have guys on the roster like Mingo and Flournoy that we'd like to look at (and their deals are not expiring yet), so they are still under our control at very reasonable coin.
In the immortal words of Bob Sugar "It ain't show friends, it's show business"