This could all be a tactic on Sprints behalf. If more wireless providers back out, than the cable operators may not want to sell as much of their spectrum. If that were the case that would really hurt T-Mobile since they are the one that really needs it.
Now while I wish Sprint would go in the auction if not just to up the price, I do believe this is a smart and precise plan. While they are somewhat limited on low band spectrum, as others know the 600MHz won't be useful for at least a couple years. Why spend millions of dollars now on something you can't use when you can spend it on building out your network and strengthening your infrastructure.
I approve Sprint, you may now go forward with this plan and become the monster we know you're capable of.