Call&Go
Carry two mobiles everywhere? Not me, I Call&Go.
These sorts of deals won't work until the networks offer two lines on one handset (yes, it's in the GSM spec).
I want to keep my business calls away from my personal calls, and 2 lines is the perfect way to segregate the ownership. Company pays the bill for the phone and business calls. Calls on the second line are charged directly to the employee. From a network perspective, this is likely to increase revenue - if I have a work and a personal mobile, they are likely to be on different networks. If it's two numbers on one SIM, they're on the same network - network wins!
I guess there aren't many handsets actually support it these days :(
And yes, I know, you get dual SIM contraptions - but that's not the same as it just working as per the spec, is it!