[Retr0id]

I don't see why you'd actually need any SIMs in the first place if you wanted to DoS a cell tower. My guess is that it's basically just a device farm for either sending spam or receiving activation codes for spam accounts elsewhere. By putting them in a populated area, the increase in traffic is less noticeable.

[jacquesm]

It makes it much harder to nail down exactly where the farm is. You can't just go break down all the doors in a large high-rise and the reflections of the radio signals in the urban canyon will further hamper your ability to pin-point the devices. But you might be able to correlate power consumption or heat signature with activity.