Kinda sorta.
Ghosting is a problem with a keyboard registering more than certain number of keys at once. Usually it's around 3 or 4.. So you're pressing W, strafing with D, holding down shift, and then when you press spacebar the keyboard wigs out and registers some other random keys messing everything up (IF it's plugged into the PS2 port)
If you're in the same situation but plugged into USB it will either not register anything extra or act like nothing is pressed.
IMO it's a non-issue except for old or crappy mechanical keyboards. If you have a rubber dome (logitech, microsoft, etc etc) this isn't a problem.
A lot of rubber domes don't even properly support 6KRO over USB across the entire keyboard.
It certainly is a problem... if it's a problem for the individual making the purchase.
Even a lot of "gaming-optimized matrix" boards, like some mechanical and some dome(including scissor switches), only technically can be called 2KRO.
If a board cannot even truly do 6KRO across all keys (and that is not including modifier keys), I'm pretty sure it cannot magically reach NKRO over PS2.
If it's a true NKRO board over PS2, it will have no problem maxing out USB (and some boards even have special USB signaling methods that support 10+KRO).
As to it being a problem, well... it's an individual thing. You'll know if it's an issue or not, it's not something where you don't really notice it yet it causes problems. If you press a key, unless you are mashing and aren't paying attention... you likely know what you want to happen following that key press; if it doesn't happen and you've been hitting a lot of other keys, then quite likely "ghosting" is holding you back (how much, if at all, is another argument... and once again should be something you can feel out for yourself).
However, it's not relegated only to MMOs.
Well technically, NKRO might as well basically only be advertised for an MMO and/or complex RPG/RTS player... but the difference between true 6KRO (across all keys, not just the WASD cluster) and limited 6KRO/2KRO (or to be accurate: 2KRO with select key combinations allowing up to 6KRO), can definitely impact performance in many multiplayer games.
To be fair, it's not an average occurrence in FPS games to utilize more than 4 or 5 keys at once, and likely one or two of those is a "modifier" key (like CTRL, ALT, Shift, Tab, etc)... and other keys will be one-time on top of the continually-depressed keys at that moment.
It probably happens rarely enough (unless your are a truly pro or pro-level gamer) for 99% of the general gaming community that one shouldn't make an effort to get such a keyboard (or pay much more), unless of course you notice something is seriously wrong between key presses and what is displayed.
If that's not you, purchase a board based on preferences.
For reference: I've recently gone through a few keyboards (and one arrives tomorrow), attempting to find just what I'd actually prefer most.
I had a Logitech G110 for the longest while, felt like I should try different switch types.
First I tried the Razer Lycosa, just to see how I'd react to scissor-switches on my desktop, with a full-size keyboard to boot. (yes, it's technically a dome/membrane-style key type).
Then, with enough research, I felt prepared to try a mechanical board. Because I was utilizing Best Buy so returns are super easy, I picked up the Razer BlackWidow Ultimate (I want/need backlighting!). I can get along well enough with the Cherry MX Blue switches used in this board, but prefer no click.
I then was notified by Razer that the Black Widow Ultimate Stealth Edition (Cherry MX Brown switches, matte finish as opposed to the mirror-like shine of the original BlackWidow line) was in stock on their website, as I had signed up to be notified.
It also appears a nice benefit to this upgrade is the fact that the Stealth line also features true 6KRO across the whole board. The original BW line only supports 2KRO with limited "6KRO" around the WASD cluster. I already noticed... I think... that a key press wasn't registered when I was trying to do a lot of things while running around in BF3. I can't be sure... and I cannot imagine I'll have myself in a situation where I find that to even be an issue all that often. The fact that the keyboard arriving tomorrow improves the KRO figure is a nice bonus but I seriously doubt it'll hamper my abilities all that often.
Granted, I'm no all-star, so some of the top-ranking BF3 players may have other things to say. I cannot imagine it's anything other than rare even for the top BF3 players...