Agret

Played one Day of SP then played a few hours of MP at a LAN the next day.

Maybe you don't play games much but anyone who plays games more than occassionaly are only interested in MP-Supported titles, of course I would have bought Doom 3 even if it didn't have MP beacuse it's one of THE games (Doom 3, Half-Life 2, F.E.A.R, Halo 2)

Nope you're wrong, nobody buys SP games these days (except lusers with 56k who can't play Doom3 MP anyway)

there is already a co-op mod out (it's not very far in production i must warn)

Fileplanet and d3files.com are the trusted sites. Don't download from any other locations if you want to stay secure.