I think the Knicks should start Quickley - if he's the best they've got... you pencil in your top 5 players at each position as your starters, do you not? (and he clearly has been the best PG in preseason play)
The only general exception to that rule is if you have a selfish killer aggressive player with a #1 scoring option mentality... that isn't quite franchise material. Then you use them as your 6th man.
The main downside is that you hurt Payton and DSJ's feelings. But Payton's dealt with disappointment before and the message to him is clear that he's not a star (he's a former lotto player on his 4th team that's basically playing close to a min contract!). DSJ... well, he needs to go there's no future for him here, unfortunately.
The upside is that we potentially found our PG of the future, and he didn't even play PG at the college level!