Interoperability
In a public network environment, products from different vendors need to operate with each other if voice over IP is to become common among users.
I believe SIP, and H.323 to a lesser extent, are used extensively in enterprise deployments, but it's sad that all the popular consumer stuff is based on proprietary protocols and essentially does not interoperate.
That said, a few times I've been invited to calls with a SIP URI, and pleasantly surprised that I didn't need to download yet another several-hundred-MB app just to join.