I think there are two questions that you should ask yourself with Jimmy Butler.
1) Is he going to improve even further and if so, what is that ceiling?
2) Although he's not a franchise player, would he become (or is he already) good enough to be the 2nd or 3rd best player on a championship team; ie Ray Allen on the 2008 team.
if the answer is yes to both of the above, you'd have to be seriously considering offering a max contract to him.