With extra parameters, search engines can index the same content under multiple URLs, which may split link equity. Vanessa also mentions that indexing parameters can hurt crawl efficiency as well as display and branding.
Seven months ago, Google, Yahoo and Microsoft (at the time) signed on to create a canonical URL tag, to indicate what URL is the “correct” version, without parameters. In her article, Vanessa compares these and several other methods of eliminating search engine performance interference from parameters—and each has its drawbacks. The new Webmaster Tools option, for example, might be used to accidentally close off portions of your site to search engines, and only works for Google.
On the other hand, although the canonical URL tag requires the page to be crawled before it can be indexed, and might thus cost a little in initial crawl efficiency, for most cases, it will probably be the easiest to implement and most universal solution. Like the Webmaster Tools option, it can be done incorrectly (Vanessa says some sites accidentally indicate the canonical URL for each page on the site is the home page). Currently, she says, Google is the only search engine actively using the tag anyway—so at present, the universal benefit is moot.
What do you think? Will you use Webmaster Tools to strip parameters from your URL, the canonical tag, 301 redirects, or another method? Or do you see little harm from extra parameters in your URL?