for Jabari it actually make sense to stay 1 more year
1. for next year he will most likely be #1 pick over all instead of #2 or #3 pick
I get all the other points, but in doing this, he would take a big risk (injury, misfortune, public perception issues that may or may not arise) to go from 2014 #2 ($4,108,800, $4,293,700, $4,478,600 = $12,881,100) to 2015 #1 ($4,753,000, $4,966,800, $5,180,700 = 14,900,500); a 3 year difference of $2,019,400 or about 673K per year.
that is not a huge difference per year, and it is also adding an extra year of making 0 dollars, and pushes the date back (if he really ends up being as good as advertised) for when he can get the max contract (8 figures) and potentially reduces his lifetime earning potential (more years working = more lifetime earnings)
then consider other factors beyond just injury, what if he doesn't improve in his 2nd year by much? his ceiling would drop considerbly, would be a year older, and the less mystery there is about you, sometimes equals less hype you'll get and less risk the owners will take on you.
not saying he wouldn't benefit from another year of college, but it would a gigantic risk to pass up a guaranteed ~13 million dollars to go back to college (which with his NBA salary he could do anytime, when and where he wanted) and potentially not improve, get injured, fall out of the spotlight, or some other detrimental factor. Hell, even if he plays well enough to maintain the #2 spot in 2015, he'd still have just passed up a $4M payday, and can pencil in earning $4M less than he potentially could have in his lifetime. That is a monumental decision.
Look at harrison barnes, I saw him some mocks to go as high as #2 in 2011, but decided to stay in school, and was then drafted #7 in 2012. By returning to school, he theoretically cost himself $4.7M over 3 years, not to mention the year without pay that he could've made $3.8M.
Big risk.