Vivas work,they are just expensive. The alternatives are cheaper but don't work.

Of course it could be that different people have different goals in mind. Some just want the diploma, others want to master the subject. So it depends on what the diploma is supposed to mean. If it just means: turned up then a viva is of course unnecessary and wasteful.

That would have been fun! The only time I did something like this was when I tested out of one of the intro to programming classes. I was one of the last students to talk to the professor that ran it before he left.