Jones was a non-guaranteed contract, so there was incentive to give him early burn so a quick decision could be made. He may also have earned PT in preseason in PT by playing well in training camp. Also, Felix and Burks were injured for large parts of preseason, who would have competed for the same minutes Jones did.
Murry competes for backup PG minutes with Exum and Clark. Obviously Exum is going to get a certain amount of allotted minutes for development since we put a 5th overall pick into him, but he also has to have a backcourt mate. I'm fairly certain Clark was probably on level footing with Murry to being the season. Quinn has no attachment to either player (for instance, Quinn didn't play Evans many minutes over Novak), so I believe Clark just outplayed Murry and earned his PT.