the majority of users at our site would prefer to disable this.
could it be made into a config option?

Not very likely, we are fairly opinionated that "Options" (and the cost of maintaining/supporting/documenting) are very bad and are detrimental to a project the size of Phabricator and avoid them as much as possible. In cases like this, recommend a local patch or fork.

Is there any reasoning as to how come they would want to disable it?

In D10457#94338, @btrahan wrote:

Is there any reasoning as to how come they would want to disable it?

the count is transient data that has no bearing on
what the page contains, page titles should describe the page.
if you've got a few tabs open, you only see the first few chars of each title,
which should be something that uniquely identifies what's in the tab,
e.g. task/diff ID, what was previously first, and follows the Notifications.