All three are definitely potential keepers. Though I am fairly sure that Kemp will lose his rookie status this season, and Delmon MAY come close.
Hermida has loads of potential. The number four prospect (BA) entering the season, he was ready to rock in Florida. Many considered him to be a frontrunner for NL ROY, but a little struggle early and then an injury have caused his numbers to be down. He does everything well - he's a ballplayer. He could break out next season, and with his size, strength, and speed, he could put up .300-20-20, and that's being conservative.
Kemp is a stud as well. He has all the tools. He can run, rake, and hit it far. He's 6'3", 230, and jacked. Once his eye improves and he makes more contact, he will be a stud at the Big league level. Pretty good bet to reach 30 homers and 20 steals. Carlos Beltran?
Butler is the second best hitter in the minors right now, behind Delmon. You could toss him in the Majors and he'd hit over .300. And he's just 20. Initially compared to Mike Sweeney, I think it's safe to say he's surpassed those expectations with his excellent 20 year old season. After a 2005 season in which he put up an OPS well over 1 at A+ as a 19, he has continued to develop at AA. Though his OPS is down to a "meager" .850, he continues to make hard contact everytime he hits the ball. As a power hitter, he could certainly afford to walk more (34 in 400 ABs), but he strikes out at a ridiculously low rate (61 Ks - just 15%). It may not seem ridiculously low, but for a 20 year old power hitter in AA, it is. He has advanced pitch recognition and can hit any pitch hard all over the field. Right now, it seems experience and power development are all we are waiting for. He gets "shafted" in the rankings because of his defense, but has made significant strides this season. He will be a corner OF, and has improved his defense to average. He can't run a lick, so his range will be limited, but he'll catch what he can get to and has a slightly above average arm. He should be up mid-to-late 2007, and hitting with Gordon should make that a deadly 3-4 combo for years to come.
I think Delmon will be the most valuable player of the group, fantasy and real life. I think Kemp and Butler will be behind him, as Kemp could be a five-tooler with big numbers and Butler could be a batting champ with 30+ homer power. I love Hermida as a player - his defense, arm, hustle - just his pure baseball ability. But I think he falls slightly behind the other three in terms of fantasy value. He'll be more valuable than Kemp and Butler in 2007, but after that, I'm not sure. Of course, if he further develops into his 6'4" frame and starts hitting 30 homers annually, I guess .310+-30-15 could be rather valuable
I think all four will be keepable by the end of 2008, if not by their current stats but by their potential.
PS - I believe that if BA did their rankings today and Hermida and Kemp could be included, their top 10 would be:
This is assuming Kendrick, Drew, and Milledge could not be ranked. (I know Hermida has no chance, I just did it for affect)