"Fend off" means preventing Intel from taking additional Enterprise marketshare. If Penryn is just as good as Barcelona, AMD loses. Many OEM's have switched back to Intel and I doubt they're going to switch their volume 1 & 2U servers to Barcelona if Penryn is just as good.
Everyone likes to tout performance per watt, but I can tell you that the majority of companies buying care very little about that. Performance per watt doesn't really come into play until you hit the larger server farms and datacenters (google, MSN, akami, yahoo, etc) Are they big customers? Sure. Do they make up the majority of the enterprise market? No. The rest care about performance and price... plain and simple. I'm not seeing how AMD is going to compete profitably in those sectors.