I think people are over analyzing the whole Boras angle and the fact that he is only a #2/3. Appel can't afford to not sign in a 2nd draft. It's true that he might be gone in 6 years but that is the pretty common among above average players. And honestly I'll worry about 2020 in 2019 instead of 2013.
Appel is exactly what the twins need. They need above average starters that can be in the majors fast. IF he's the best available player then you take him.
As far as I know, Appel will be the most major league pitcher available. Now that can change between now and next June. He has no leverage, if he declines he will be a 23-24 year old in indy ball who has passed on two top 10 drafts. And if by some chance he does you get a 5th pick the following year. If he is gone in 6 years so be it, thats 6 years with a better pitcher then who was on the staff this year.
The only complaint that is legit is whether or not he is healthy/ has he been over worked in college. I'll leave that to the scouts to decide.