There's no way we can move Folty to the pen at this point given what he's flashed at times as a starter. Banuelos and Hursh, sure. But Folty should be given every chance to succeed in the rotation.
I'd love to see him make it as a starter as well, but what he's "flashed" is that he's extremely hittable when he throws anything other than gas. His WHIP isn't 1.56 because he's walking the world (like our bullpen is) - he's only given up less than 5 hits in 2 of his 9 starts, and he's averaging less than 6 IP per start. If you throw out the one good start he had against the Brewers, he's given up 61 hits in 44.1 IP. If you take the Milwaukee and San Francisco starts (his two good ones) away, he HASN'T allowed less than 5 hits in a start and has given up 56 hits in 37.2 IP. His secondary pitches simply aren't ready for primetime. His overall strike/ball ratio is bad (531/313), and everything he throws other than his fastball is getting clobbered.
We all understand the upside if he can ever develop at least one solid secondary pitch, but he's going to continue to get crushed at this level until he does. You can already see he's lost in his reactions. If they're going to try to continue developing him as a starter, he needs to go back to Gwinnett and throw nothing but offspeed stuff before he loses confidence in himself.