He has never been a good ML hitter, Herrera has had the same MO each year, a really long cold streak, a really long hot streak, but when these streaks happen change year to year.
It comes down to timing, when he's on, he's unpitchable, when he's off, he's below replacement level.
I think it comes down to an exaggerated timing mechanism with that leg kick and wide open stance, as long as it was working I think they tried to leave it alone except for encouraging him to be more patient (and when he was, he put up great numbers). However, after last season I think they're going to want to rework his swing or get rid of him, I think ML pitchers have him figured out and he can't adjust.
He has good gap power, great eye to hand coordination, if he can rework his swing into something more compact that let's him wait longer on pitches, and thus learns to be less aggressive and focus on pitches in the K-zone, he could win a batting championship. If he can't, he's going to spend his career as a tease who never puts it together for more than short stretches.