Willis plays because he has a skill that really complements the rest of the main guys.
With three guards that can penetrate, you need a guy on the perimeter that can spot up and knock it down consistently. This will help keep the defense from packing it in, which will make it easier for the guards to score at the rim.
With an effective big inside, we need a guy that can keep the opposing team's 4 man out of the paint to give Bam room to operate inside.
I won't say he starts, but worst case, he is a 20+mpg contributor because of his shot, IMHO.
Now if Fox, Monk, and one of Briscoe or Gabriel shoot it well enough to keep the defenses from packing it in, then Willis might not play as much.
I just see Willis as bringing a valuable skill to the table that no one else has, at least not at his level anyway.