Chrome Version : 20.0.1132.17 (Official Build 138701) beta
This "feature" seems to have been around for a while though.
First of all, the vast majority of automatically added search engines are completely useless.
For example, I just finished clearing out a ton of search engines that I unwittingly picked up while browsing themes on wordpress.com. Each theme preview had its own domain, which resulted in a separate search engine for each theme. All of these search engines are completely useless to every single Chrome user, bar none. There are plenty of other examples of useless search engines. Do I really need a search engine for my personal blog to be part of my browser?
This feature is a major annoyance, because adds clutter my search engine list every time I simply visit a site, even when I have no intention of adding that site's search to my list. Because of this behavior, I have to periodically excoriate unwanted search engines from my search engine list. There's really no reason that this maintenance task should exist.
Furthermore, the user cannot even tell when a site has been added to his list, because Chrome gives no indication that it is doing this. Btw, there should be no such indication, as it would be a distraction in the vast majority of cases.
Furthermore, it doesn't really save typing, since the default search engine key is just the domain of the site itself. Sure, it saves a modicum of time by not requiring the user to wait for the search page to load before he can enter his query, but that assumes that he's actually aware that he even has the search engine in the first place, which as I said in the previous paragraph, is vanishingly unlikely.
Even if he does notice that an automatically added search engine is in his list (by going into his settings, and scanning his search engines), he's not likely to remember that it is there, because he didn't add it himself.
Perhaps worst of all, this is privacy hazard, because it leaves a trail of search engines that a) does not get cleared from the browser's search history b) the user is most likely not even aware of.
Here's what should actually happen: when you visit a site that has search functionality, put the site in a list of "uncurated" search engines. That way, the user can discover such search engines later. At the same time, his manually curated search list remains uncluttered. Presumably, this is how users discover search engines in current versions anyway.
Finally, having an uncurrated list also makes it possible to clear automatically added search engines from the browser's history, greatly decreasing the privacy hazard.