In your application you may wish to add authorization to your writeURL documents. Writely does not provide user authorization. It would add way way too much complexity to their system. There is no need for them to know who is using your documents. You want to maintain as much privacy as possible. But some users, myself included would like to add authorization. It turns out is quite easy to plug them into my authorization system.
My plan is to have my clients connect to my server, where i authorize them, and them pass them onto the WriteURL server. My clients will never see the secret write URL. It gets attached to the messages only on my server. In my case I am okay sharing the read URL's. You may want to authorize both read and write access. I can easily invoke all of my own authorization machinery, while still benefiting from the wonderful reusable WriteURL components.