This is a trend that should end, but you never know. Many of these guys will go fouth round or lower, if for no other reason than there are just to many of them. While it makes sense that they want to get the clock started for their next contract, the fact is that leaving early for most of them will mean they may never get to a second contract.
Sadly, NFL teams will give higher draft picks a much longer leash and more opportunities to succeed because of how much more they perceive their investment in a higher draft pick. So, leaving early and being drafted later means teams are less likely to give you the opportunity to succeed.
In my opinion, unless you are a guaranteed top 15 draft pick you should stay in school. Why 15? Because lower than that and you could drop out of the first round, which is not unusual. From a Michigan perspective, Donovan Warren is a great example, if he stays he probably goes in the top 3 rounds and at least gets a chance on an NFL roster.
On a different subject, I find it funny that so many people think the rookie scale is wrong because athletes that don't ultimately play well in the NFL may not get a big pay day. To me it seems like the perfect system. If you actually make it to the NFL and play well you will get a big pay day, if you don't you won't. Granted some that get injured may lose out and I feel bad for them, but ultimately you are still talking about someone not getting paid for not being on the job. If they were smart they got a degree in college and have a ton of opportunities left.