It's fucking stupid. It's rarely needed function that is rarely used. IT'S STILL NEEDED.
I use "close tabs to right" all the time, myself, so I hope this doesn't go away. That said, I disagree with your idea that if something is rarely used it should be kept. Your other example (clearing cookies) is a bad one because there is no other way to do that, but in this case tabs can be -- and generally are -- closed one at a time, and in fact Chrome is careful to move the tabs around so that the close button for the next tab is under your cursor when you close one. This means that "close to the right" can also be done by moving your mouse to the "x" on the first tab to remove, then tapping the mouse button rapidly until all of them are gone. Unless you have more than the 20-30 tabs that I typically have open, that's really not so terrible. Plus, as others in this thread have pointed out, you can multi-select tabs then batch close them that way. There are other reasonably-good ways to achieve the goal, so if this is one is rarely used, there's no way to argue that it's actually necessary.
As for why to remove it... features cost. Every feature you keep in a product is a feature that has to be maintained and tested. Development and testing resources are not infinite -- not even at Google -- and the accumulated burden of lots of old and rarely-used features gradually slows progress on new features, security fixes, etc. It makes a great deal of sense to remove features that aren't used much and which have more often-used alternatives.
 My normal browsing style is to open every link in a new tab, and to use Ctrl-W to go "back". So my tab bar ends up being a breadcrumb trail of my path through a web site, and when I'm done with something I close the "site" with "close tabs to right". I also keep a couple of pinned tabs (email and calendar, in that order), so when I want to close "everything" I've been doing, I "close tabs to right" on the calendar tab.