ESN swap restarts 2-yr rebate eligibility?

I have an old Sanyo 4900 that is giving me problems lately. I've been out of contract for a while now.

My mother in law has an extra Sanyo 7300 that I thought I would activate on my account just for now, until I have some time to get to the store and make a decision on what phone I really want. My plan is over $34.99 etc so I should be eligible for the $150 with new 2-yr contract.

I was all set to log on to sprint.com and do the ESN swap but thought I'd check here first to see what people have to say about the new phones that are out etc.

I was surprised to see this comment in the Sprint FAQ:
*If the phone has not been upgraded since 9/06 the rebate eligibility will be determined by the last time a new esn was placed on the account, even if the phone was purchased at full price or acquired from an outside source. Warranty replacements with reconditioned phones will not effect eligibility.

This makes absolutely no sense to me at all -- is this really the policy? What is their justification for this?