There is so many things wrong in this thread, it makes my face hurt.
#1. XBL allows for normal, dedicated servers. L4D and L4D2 uses them. So does MMOs and, I'm sure, other games. This is not the issue.
#2. The PS3 doesn't have 7 cores. It has ONE, uno, ichi... ONE CPU core. It has 7 SPEs. SPEs are not CPU cores, nor do they work like a CPU core. Read up. This is not the issue either.
#3. It has nothing to do with the 360's design. If my 2004 Mid-rang PC can run MMOs and other massive games with little lag, the 360 should be able to do it lag-free. There is nothing, and never has been anything, keeping one system from running games similar to the other systems games. This goes for PS3's Killzone 2 and 360's Gears of War 2. Both can be had on each system, they arn't as mind blowing as you think once you remove your fanboy glasses. This is not the issue, period.
What is the issue? Money. No one developing for the 360 sees a game like MAG as being viable. After getting sick of the beta, I can tell you it's just another sub-standard shooter. Unless they come out with some really mind-blowing playlist or whatever for the full release, it's going to be a clusterfink of epic proportions lead by every 12 year old on PSN. The only reason we don't see this in the beta is because most kids couldn't talk their parents into the Qore subscription. It's turning into another Killzone 2 launch. It's going to be hailed as the savior of the PS3 by the fanboys, then it will fall faster than the nickers of a drunken prostitute... but I do enjoy my Killzone more than MAG. I don't like being screamed at by some fratboy with a Napoleon complex.
It all comes down to money, money, money, and more money. What doesn't the 360 have wireless? Money, Money, Money. Why does the games for all systems cost so much? Money Money Money. It's what the world runs on.