In the end, it all depends on the concept you're talking about.. When arguing exclusives, it needs to be clear what the standpoint is.
Usually, it's about the CONSOLE war.. The PS3 vs the X360. The PC is not a console and simply doesn't take part in the console war. The Wii is obviously a different audience so I left that one out.
A)
From this perspective, it's simply not relevant if a game is on the pc or not. But when talking about the console war, people like to say "The X360 has no exclusives because the games are on pc as well, and therefore the PS3 beats the X360". Then I ask myself, where is the PC then? Is the PS3 also better than that because it also has no exclusives since you can play them all on your X360 too? That it's on the PC is not relevant, because again, you're talking about the CONSOLE war. In this case, you have only two candidates, the PS3 and the X360, and, in this case, whether it's on the PC or not, they are simply X360 exclusives, because the PS3 does not have them. Two candidates, one down, one left, therefore, exclusive. Period.
B)
It can be viewed from a different perspective as well. If you're looking on ALL the options to buy your games and you have a pc that can run them, then the games on the X360 and the PC are not exclusives. But in this case, you can not say that the PS3 is beating the X360 because it has more exclusives, and at the same time praise the PC, while the games on the PC can be played on the X360 as well.. Because you are then praising one platform (PC), and bashing the other (X360), for the exact same thing that they have in common, and thus you are contradicting yourself. You can only argue that you don't need an X360 to play the game, but that does not transfer into making the PS3 better because of more exclusives, because you are then back at perspective A.
You either look at all 3 without praising/bashing any of those, or you simply forget the PC completely.