I think you may have misunderstood the way the header works.
X-XSS-Protection basically provides a way to the browser "if anything on this page looks suspicious, don't run it" (either the whole page or just the bit that looks suspicious). It isn't supported by all browsers (Firefox, for example, doesn't support it).
In theory it's a reasonable idea, although a pretty weak protection - it only benefits the users of those browsers from being victims of XSS attacks if your server has already been compromised. Web applications therefore need to take much more sensible measures against XSS protection such as ensuring proper escaping of output (to stop them happening at all), http-only cookies (so even if there is an XSS exploitation it can't access your cookies), etc. We do all of this.
So in other words: all it provides is a very weak level of protection against something the backend already has much better protection for. And, as @Makoto points out, it kind of sucks at doing even that; it is known to have bugs and ironically, some of those bugs cause security issues themselves. Also, there are known ways to bypass it. That's probably why some browsers don't even support it.
Normally, it would barely be worth any thought and we would leave it at the default value. But it was breaking things with false-positives (i.e. it was thinking that code we deliberately wanted to run was suspicious) so turned it off. Apparently we are not alone in going for this option: I just quickly checked Google and Facebook, and both have it turned off (full disclosure: the other two sites I checked, Twitter and Amazon, don't).
You can turn it back online with a plugin or via your server configuration if you really want to, and it's also possible that the Content-Security-Policy header which we do have a setting for will override it (you'll have to check each browser), but we're not going to add a setting specifically for it.
tldr: It's a thing that isn't supported by all browsers, with a much grander sounding name than it deserves, which is buggy, and was breaking things. You don't need it on.