At this point, most SEOs and Webmasters would agree that Google Webmaster Tools is an invaluable resource for any web site on the internet.
Even if you do not use it for submitting and tracking sitemaps, it can be so helpful in many other ways.
That being said, some are surprised that only a limited number of sites backed by Y Combinator, a venture fund which focuses on seed investments to startup companies, uses it on their properties.
Google's Matt Cutts wrote in a HackerNews (a Y Combinator property) thread:
The list includes Dropbox, Scribd, WePay, OMGPOP, Loopt, and Weebly, but a lot of sites are missing out on potentially useful information--only 18% of 136 sites were detected as signed up with webmaster tools.
You can see the list over here.
The discussion gets more interesting in the HackerNews thread where some people suspect the 18% figure is lower than it should be.
Forum discussion at HackerNews.