There is an advantage, which is that knowing your board without the labels makes you a better typist. It is hard to explain. But let me ask you this: why aren't there note labels on piano keys? Would you have better piano players if they had labels, and they would not be able to play them without them? I think their control would not be as good. Feel the board, Luke..Be the board! There is no looking.
If someone came over night into my house and engraved Binary lettering, Klingon, Japanese or Sankrist on my keyboard it would've probably taken me quite some time to notice it since it won't affect my typing whatsoever. I have gained the required muscle memory to type in the languages I can communicate in, so I can and will utilize a keyboard for its purpose with no regard whatsoever to the lettering on the keyboard. A blank keyboard maybe preferable for someone to obtain the aforementioned muscle memory, but for someone who has already achieved it, it serves nothing but an aesthetic preference.
You won't have better piano players if you mark the keys with their notes, but if you sit down an already trained pianist in front of a piano with funky misplaced notes he will have absolutely no trouble playing it just as well as a "blank" piano..
The only OTHER one I can think of is that a lot of people get PO'd that the lettering eventually rubs off or discolors on modern keys (anything that isn't dual shot or dye impregnated). You avoid THAT problem but still end up getting shiny keys eventually.
And the Piano analogy is OK but not perfect. You've got a linear progression of notes, black keys to "signal" octave boundaries.
Maybe a cello would be a better analogy.
Neither are the Piano or Cello analogies work since both are musical instruments, you don't press random keys but go up and down a musical ladder. Even my 2 year old cousin knows that if the key to the left of the one she's currently punching at makes a lower sound, then the key further to the left will make an even lower sound. Not only do you get an audio feedback, but its "relativity" remains constant. While you do, usually*, get a visual feedback from your keyboard whether you're looking at it or not (since the letters magically appear on your screen, if they're not, your keyboard isn't fulfilling its function), there is absolutely no constant,
relative, relation between your input and the feedback that you're receiving.
*Quite often at work I need to feed data (be it by numbers or letters) into the computer from a sheet of paper, a closed network computer's monitor, or anything else "outside" the system I'm working on. I can rely on my muscle memory to touch-type entire pages/spread sheets without ever needing to verify mi-way that I'm typing the right data into the right place. This is more of an efficiency choice than a necessity, and I do admit that I'm confident enough to state that the random interchangings of two letters have more to do with the nature of the keyboards I'm working with (robber dome FTW), than with my typing.
Anyhow, I do admit that the aesthetics of a blank keyboard serve as quite a nice "plus", be it the simple awesomeness of a blank keyboard or the lack of need to worry about keys robbing off, but I do have trouble justifying the purchase of a blank keyboard that will make it very difficult on people who don't touch-type to use my computer. I will be getting my first mechanical keyboard soon, an FKBN104M/EB (which I've queried about some 3 months ago, heh), so I just might justify to myself getting a blank keyboard as a smaller more awesome brother. But I will never fool myself into thinking that it has something to do with anything but aesthetics.
