What He's Done To Get Picked Up in Fantasy:
He's gotten hits in the last three games and thrown in a balance of power and speed in the process. Also, having the batting average over .300 (.304), with some speed, a little pop in his bat (all while having eligibility at second base) along with third base and outfield.
What To Expect Going Forward:
While he has been one of the few players that has performed strongly against the Brewers, he doesn't get to play all his games against them. One encouraging sign of progress is that he has improved his walk rate over his previous years in the majors. I think his batting average will see some regression to the .270-.280 range (his .333 BABIP is the highest he's posted in the majors so far) and I could see him reaching double figures for homers and steals assuming that he continues to find playing time.
Overall, Harrison seems to have made progress this year compared to 2011-2013 as this is his first year he's on track to have an above average OPS+. The key with him is does he continue to get consistent playing time and how this gets impacted if he struggles for a little bit? He's definitely worth looking into due to being eligible in the wasteland that is second base but he would probably have to demonstrate a continued improvement in plate discipline to continuing to improve overall performance. If he wasn't already picked up in my league, I would have given serious consideration to adding him as I was in the search for a second baseman due to Brett Lawrie getting injured. For the time being, I had to settle for Gordan Beckham.
The Pirates have worked their way to .500 and are currently three games out of the last wild card spot. If they want to make a push for a repeat playoff appearance, they will likely need Harrison to continue to hit very strongly regardless of what role that happens to be in (along with an improvement in starting pitching).
Among players drafted in the sixth round of the 2008 MLB Draft, he currently leads in WAR (3.4) and if he continues improving he should further solidify this position.
All numbers referenced found on fangraphs.com or baseball-reference.com