WordPress.com is a multi-user platform which means we all share the same underlying theme and core wordpress files, and any bad code that might be introduced by someone, whether on purpose or not, could cause a lot of sites here to go down. I probably repair an average of a self-hosted site a month where someone has installed some nifty, "Ooooo, look at the pretty colors" code and lost both the public and the dashboard side of their site. There is a lot of bad code out there, some of it on purpose, and some of it just because the person who wrote it does not have a good enough understanding of the code they are using.
If you want to hack and use PHP or javascipt, or whatever, wordpress.com simply is not for you.
Buy a domain name if you do not have one and hire a web host. Figure, all things considered, an average cost of $10 to $25 per month depending on your requirements.