That's called pasivity, whereas Imax seems aggressive.
Its still a question but why "force" Fields knowing what he brings?
Hypothetically picture Fields on one of these playoff teams
CHI, BOS, MIA, MEM, DAL, OKC, etc...
All those teams have better sg's, and Fields would be a bench player on those squads.
Now, replacing any of those teams sg's with Fields, and we WON'T see a major drop off because Fields is a good all-around player, but still not a starter.
Now picture Fields playing defense against those playoff teams sg's... he can't be relied on to shut any of them down.
So in conclusion, Fields is a starter by default. If Shum continues to impress, moving him into the starting rotation is better sooner than later.
All we need to see is Imax vs Rose, Rondo, etc... he already held his own vs D. Williams.