In Valentine's case, I've seen the argument that declaring now and trying to improve your skills is the best long-term plan as he will play out his rookie contract sooner while also receiving better coaching. However, I think a better plan would have been for him to try to use his starter status on a high profile college team to try to vault himself into the top 3 rounds in the 2017 draft. Essentially, he would forgo approximately a $50k signing bonus and a $400k salary for a chance to get $700k+ in signing bonuses and potentially $1m contract.
Or, another way to put it would be to say, which is the more likely scenario? Valentine is drafted in the later rounds of the 2016 draft (or signed as a free agent) and performs so well that he becomes a top free agent when his contract runs out -OR- Valentine puts together a great senior season, earns all-conference recognition, and is drafted in round 3 of the 2017 draft?