It can keep FILES updated, but when it comes to the database, you're talking a live environment that's always changing. Take a forum for example. If someone logs in on Server A, and posts something, but Server B has a different user posting at the same time, there's no way to copy from one database to the other without losing someones information.
Same goes with products online. If they purchase a product on Server A, and it's not updated on Server B, then you're out of sync in a big way!
Some places have remote machines that you can browse the various products etc on the remote machines, but then once you go into the CART CHECKOUT mode, you're sent to a single machine. This works pretty good. Then you just need to update the catalog any time you are out of product. Works good if you have multiple items of the same product, but if you're dealing with limited quantity, if two people buy it but you only have ONE, you're going to have a pissed off customer.
So RSYNC is great for files, but on a database level, it's not really designed for something like that.