BCA2320 follow up question: PreferHT vmx setting

First, let me say I really enjoyed most of the BCA2320 presentation at VMworld. I've seen Jeff present numerous times and always found him entertaining and educational. This was (I think) my first time seeing Sam present and it was also excellent.

After seing the presentation and then reviewing it this past weekend, I've got a number of follow up questions.

Second one:  On Slide 54, one of the best practices is setting PreferHT=TRUE in the .vmx file and this is then explained in further detail in slides 56 and 57.

I'm confused why this would be needed and if it's a corner case.

I thought that in vSphere 4 and even more so with vSphere 5 (with vNUMA) ESX tries to schedule cores to take advantage of NUMA. So wouldn't threads/cores on the same socket CPU be utilized even without the PreferHT command?

On a more general note, what performance improvement have you seen with vNUMA in vSphere 5?