For example with HSPA+/LTE modems, to have proper OOB into the routers at a crucial POP.
$6/mo per active SIM card is pretty cheap for M2M data SIMs, though the $/MB rate is not the best. But for the application I have in mind it would be console SSH traffic, which is super low bandwidth. https://ting.com/rates For LTE they're an MVNO on AT&T and T-Mobile. Looking at the FAQ they say that the data usage can be limited and monitored on a per-device basis, which could be useful.
