Because there is a business need.
I recommend a client never take on a technology unless there is a need for it, and even though there is a lot of promise in Cloud technology, the same rules apply... there has to be a business need or it won't pay. Ecommerce and some of the other disruptive technologies worked the same way - if you did Ecommerce for the sake of a sizzling new technology, things generally didn't turn out so well. If you did it to gain market share, optimize processes that use more manual methods/paper methods, etc, you saw real value.
So what business value does public cloud provide? The question I come to is what services are offered today or could be offered in the future (in line with the business of SPs) that would be well served with Public cloud technology? And there are alot of different types of SPs too, so just to narrow it down, let's talk about telecom companies. Do the AT&Ts and the Verizons need it? or can the Tier 2 providers benefit more from it by providing services more quickly, more economically and more efficiently? Thoughts?