This thread could get fun quickly.

Chip's right of course regarding innovation and I am sure there are people who do support their own installation using modified core files. If you want to learn or scratch your own itch then go at it.

But from a support standpoint it's a horrible idea. Given that (while not always straight forward and easy) WordPress is extendable via plugins, theme functions.php, etc. modifying the core files also means you are swimming in the deep end of the pool alone and by yourself.