I think one factor is larger bullpens. Teams carry too many relievers and don't carry that extra bench guy. These days, a bench player has to be a multiple position guy who can fit in a few different roles and is often a guy who doesn't have the bat to start everyday. The 12th pitcher role could just as well be a hitter who can hit the ball out of the park. Personally, I would rather have a guy who can hit the ball out of the park instead of another bullpen arm. I would just use the other 11 arms differently but that is just me.
Secondly, I think they are under appreciated. They are not "profile" players because they have to face righties and they often aren't well rounded players. Mark Trumbo was written off as a slugger that didn't profile anywhere but 1B and would really have to hit. Well, give him 600 PA's and he really hit. Paul Goldschmidt is another example. He has actually been a better defender and more athletic than many expected from his minor league numbers.
Now that random second baseman aren't jumping from 12 HR's early in their career to 37 later in their career like they were a decade ago, players who can hit the ball out of the park are getting more valuable and I think the guys like Trumbo or even a Mike Hessman type of player will start getting more opportunities to show if they can hit the ball out of the park enough while not making too many unproductive outs. Pretty sure that should have been more than one sentence...
IF a player can hit .260/.330/.460, that is a valuable player. That's not a star but it's a valuable MLB bat, especially if they have any value anywhere else, defensively or on the basepaths.
I don't know if Darin Ruf has figured anything out or if it's a fluke but hitting 38 homers in AA isn't easy. He only hit 17 last season in the FLorida State league but if he starts hitting similarly in AA next season, I don't see any reason for a MLB team to give him a shot. Not like 50-60 PA's. I mean a few hundred to see if there is any MLB translation. Sometimes scouts are wrong and sometimes a player that isn't a prototypical prospect in the minors can make it work in the majors. I'd love to see a team like the Astros give him a shot. Brett Wallace isn't working, why should he get more chances.
Nate Frieman is another example.