I need a harddisk to play firefight online?

It's not to make money, it's to do with streaming data from the HDD. When the game has 4 players over a internet connection and has to run the enemy AI simultaneously for firefight, the dvd can't stream all this fast enough to the XBOX 360, so the game offloads some of the work to the HDD's cache which has a faster read speed than the DVD.

For multiplayer it's different because it doesn't need to run any sort of AI as everyone is player controlled and you don't need a hard drive.

But why can't I use my 16GB USB Flash Drive for streaming data.I already installed halo to my USB Flash Drive .Ok I guess they always have a reason to tell you:"the USB Device is not fast enough to do the work"

But I can use a USB Device to play world of warcarft.Does halo gets more players and NPCs than word of warcarft?

No but WOW has servers, and Halo is P2P. There is a difference there in data sent and cache stored on your Computers HDD, and xbox. It would not happen if there were dedicated servers like there are for WoW. And secondly, why in the world would you ever buy an xbox without an HDD? I get that it's cheaper, but this problem is persistent for more than one game. You should invest in a Hard drive.

Don't get too angry about it, you'll need a hard drive to do anything significant anyway.

.

Forge, other games, installations..... Watch as your 16GB disappears.

Just get a hard drive, it's worth it, trust me.