Couldnt someone make blocks very very large by doing it repeatedly?
It would take a few seconds to make a endless bash loop doing it.
I suppose that as soon as transactions fees start to be demanded, that might become "expensive" to do.
And I think the only reason such fees aren't charged right now, it's because the only client that exists doesn't give the user the option to demand them.
So, if bitcoins ever get popular, I believe all transactions would have to pay a fee. Maybe tiny, but something.
Why can you send coins to yourself?
Maybe to merge/split your money among different addresses.
Or another more practical example, suppose your laptop is stolen, and your wallet.dat is in it. If you have a backup, you'd better use it soon to transfer everything to a new address of your own, which is not on the stolen wallet.dat, before the thief does it.