I think at the time that Bioware became available for aquisition MS' EDD was 3 billion in the red from the 360 launch and RRoD costs. I'm sure that the execs felt a big push to show that this business is viable at that stage and the last thing they needed was another ~1 billion dollar expense.
At this stage EA's ROI is likely to take >5 years, with Mass Effect and Dragon Age having great commercial success but failing to wow the charts like a Halo or Call of Duty.
I feel that their current strategy is the best as it allows for the most flexibility/control and variety. At present if they see a void in their lineup say JRPG's they can partner with Mistwalker for Lost Odyssey and Blue Dragon or Tri-Ace with Star Ocean etc. I don't think there will be as much in the pot to continue with these partnerships should they spend a billion on one developer.
