* It takes bandwidth
* It's (presumably) before a pageload, and is blocking... the latency to load the original URL to get the "link" gets added to the consumer's pageload time.
* Now you have to deal with "timeouts"
Ideally, "link" should be used by the browser, and not the web service. By the way, other such auto-discovery systems are OpenSearch (first proposed by Amazon, and is pretty widely adopted) and FavIcon.
At any rate, I think tinyurl, and bit.ly, etc, are a pretty fast and easy solution at this point. They should improve their services by including a "title" attribute to the link they give you, which says the URL and/or page title it's going to.