We already have the CPU power, unfortunately this power is currently used it to take images of our private parts and send it to each other, if they could come up with a app that can use smartphones to execute a part of the program then we wouldn't need to build these.
I don't want my battery being drained even more than it already is. As it stands many modern smartphones can't make it through the day without a batter backup helping it along. No one wants thier phone being used when it's not them using it.
To paraphrase something Neil Degrass-Tyson said, taking away the funding from NASA takes away the publics ability to dream of the future.
Well I'd say this qualifies as a good stand-in.
We need this kind of computational power to learn more about the human brain.
We need this kind of computational power to simulate weather systems
We need this kind of computational power to simulate deep space travel for manned and unmanned craft.
We need this kind of computational power to simulate new energy production methods before actually attempting them.
There's countless uses for supercomputers, that's why developped nations around the world have been building them for decades.