Well, if you grew up with Mark II's (and for those who grew up with Mark I's and Standards) then it's hard to not notice a large, oblong hole cut in to the side of what was an outrageously familiar image, especially when you consider that it looked pretty nearly EXACTLY the same for fifty-five years before Ruger decided it was time to make a rectangular slot in the port side of it.
So you can "notice it" in that way. And for all the others who have REMOVED it, they probably noticed it when the pistol would fail to feed or eject cleanly, which is even more noticeable for people familiar with the legend of the pistol. Frankly... failing to feed and eject is simply something the Ruger Mark I and Mark II pistols didn't ever do much of...ever. The Mark III does it often enough (YOU decide what that number is) that a 2-minute Google search pulls up a myriad of fixes, suggestions and work-arounds to address these problems and get new owners running better.
This is probably not the best time to mention that a loaded chamber indicator is an idiotic device that promotes lazy and unsafe gun handling right at it's very core. Is a chamber loaded? Of course, because Rule#1 says that it's always loaded and you should treat it as such. If you need for that chamber to be unloaded, you ENSURE that it is, and you don't leave that necessary bit of information to an idiotic, later-added and completely loathed device such as a "loaded chamber indicator." You open the action, you look inside, perhaps you even FEEL inside and in some cases, you insert a flag to REALLY ensure that it's not a loaded chamber.
That's not something to be left to a gizmo. That gizmo is as worthwhile as an on-board or integral lock.
Attention Brass rats and other reloaders: I really need .327 Federal Magnum brass, no lot size too small. Tell me what caliber you need and I'll see what I have to swap. PM me and we'll discuss.