I agree that the situation will change if Mahinmi is able to carve out a niche and if Thomas (or Oberto) start playing better. Next to Duncan, there are about 60 minutes up for grabs.
I think Tolliver is the longest shot to win consistent playing time. Bonner has the inside track on him due to his experience alone. For the sake of the Spurs, hopefully Mahinmi wins minutes because he could provide the aspects the Spurs lack (athleticism, shot blocking, height).
We'll see, though. I think Pop will be fair and give all five of the complementary bigs enough minutes to let the cream rise to the top. Right now, Bonner is making a good case for himself. Tolliver has shown some interesting talents. KT and Oberto have been mediocre but they should hopefully get better as the season progresses.