Look at it this way, Harkless is a known quantity (he's known to be moody, mercurial and inconsistent). Terry likes to experiment with the rotations early in the season. This is a chance for Gary Trent Jr. to get some minutes so we can see if he's:
a) able to play SF, or is strictly a SG
b) if he's ready to earn rotation minutes
I view Trent Jr. as a potential Allen Crabbe replacement/upgrade. He can shoot the catch and shoot 3 like Crabbe, but he's MUCH more aggressive offensively, gets to the line and isn't afraid to take a big shot. The season Crabbe broke into the regular rotation he played more minutes at SF than at SG. We ran a lot of small ball lineups with Dame, C.J. and Crabbe all on the floor a the same time. Like Crabbe, playing next to Dame and C.J. should result in plenty of open looks for Trent Jr. I look forward to seeing how he does. If he does well, it will show Moe he's in a battle for minutes and the SF role isn't just his by default.