The problem we have on PC is that all our servers are dedicated with providers. Changing a server from Conquest 32 players to Onslaught 4 players cuts out 28 player slots from the total amount available. This could have a dramatic affect on the PC online environment if enough servers changed to Onslaught.
Honestly I'm not too disappointed that the PC version won't be getting the new mode as I play Bad Company 2 for the large scale battles and this seems to run counter to that. I'm guessing we'll see this released for the PC version at some point but I hope that it doesn't hurt the very rich, very awesome online community for the game. I'm not just saying this because I finally unlocked the last weapon in the Assault class and need some n00bs to use it on.