There's about a zillion frequency bands for 3GPP technologies these days - 700MHz, 850, 900, 1800, 2100, 2600, AWS, various Japanese ones, upcoming UHF bands and so on.
But there's some notable absences - unlicenced 2.4GHz and 5GHz bands. The ones that WiFi and Bluetooth and a million other gadgets use.
Sure, its unmanaged, congested, interference-prone.... but for short ranges, who cares? Like, for example, if you had a 2.4GHz HSPA femto at home which you could "roam" onto? (I'm assuming you wouldn't have your own SIM card & HLR).
Why would you want to do this? Different business models - lots of them. And a complete end to the need to put WiFi into phones as well, if you could have a single multi-band chip that supported private cellular as well as operator cellular.
Yes, I know it would have some horrible problems, and I certainly couldn't see the stuffy standards bodies daring to support something so lacking in QoS and control and operator involvement. But if cellular ever wants to stand a chance of competing in homes or enterprise networks, it needs to have an "owned" profile as well as a "service" profile.