All that being said, I still dont think any roster should start wth a guy who is not a matchup nightmare on offense (That's my personal criteria for who you should build a roster around.). I dont think teams will ever have to game plan or really focus on doubling Marcus even if he reaches his full offensive potential.
I'm on the opposite opinion of this. I think that if you're a liability on defense, you really shouldn't be on the floor. Isaiah is the best example of an offensive mismatch on this team because of his quickness, but we can't afford to have him start if you don't expect every guard to abuse his defense.
Like I've always said, it'll be a lot easier to improve one aspect of your game (offense) if you're already above average in another aspect. Especially for a rookie. For comparison's sake, I've lived in Oklahoma long enough to see the disaster that Russell Westbrook was on both ends of the floor during his rookie year but got by due to his elite athleticism. He's worked on his game and really hit his offensive stride during his 3rd year (though I still don't think he can shoot).
On the other hand, a kid like Damian Lillard has so much time to work on his defense to see improvement just as James Harden improved this year. There's still plenty of time for Marcus.
Back to your actual question. Would he be starting on a Banner 18 team? Absolutely. Championship teams need a player with heart. If you look at the last group of champions, there's always a glue/heart player that drives the team with defense and effort in the clutch (Draymond Green, old Tim Duncan, injured DWade, old KG, Chauncey Billups and Ben Wallace, Derek Fisher). How many plays did he make late in games, especially during the playoff stretch of last year?
If Marcus even turns out to have the above-average ceiling of a Kyle Lowry, but have the heart of KG; How could you not want that as your starting pg.