I doubt there is much difference between running your own computer or sharing a supercomputer that is roughly ten thousand times as powerful with at least ten thousand people. Not to mention the bottlenecks caused by the internet connection you need to tell it what to do, and get what it has done back from it.
Err. Not sure you grasp this.
Computer A and Computer B
Computer B is 10,000 times more powerful but is shared with 10,000 people
The key difference is timing and time. If all 10,000 people want to do the same thing that they do on their A computers on computer B at the same time then I agree with you. However that is not real life. If the distribution in time of tasks from the 10,000 users is spread out then my task will run up to 10,000 times faster on computer B than computer A.
If I could use computer B all the time there would be no point in sharing it. If I don't (and I assume all the others sharing computer B can't use it all the time either) then there is a benefit to me using computer B since my task will complete faster, maybe much faster.
As far as internet traffic is concerned I assume that the task I want to do on Computer B is compute intensive on local data, like ray tracing / animation.