That's what you're missing, I'm not saying he'll play 40 minutes, or 20 minutes each half. He's the second option behind Reese, so if he misses 15 minutes in the first half because of foul trouble, he's only still gonna see 10-12 minutes in the second half because he likely won't play over Reese or Moore if we go 5 guard.
If he plays 5 minutes in the first 10 minutes of the first half and only has 1 foul, that allows Barnes to go to him again in the first half, but often we've seen McGhee pick up 2 fouls in the first half and as that means with most coaches/players you're done for the first half, and that's almost always when we've seen Alexander/Kasongo/Schofield...late in the first half with McGhee on the bench with 2 fouls.