Why superimposing PET data on MRI scan images instead of doing fMRI scans? I'm currently reading this paper: 

Savic &amp; Lindström (2008). PET and MRI show differences in cerebral asymmetry and functional connectivity between homo- and heterosexual subjects.

It certainly is an interesting study but I just can't understand what the benefit is of mapping PET data on MRI scan images instead of doing fMRI scans. Any ideas what the benefits are?

Also, I found out that reference 64 is referenced in the text but missing in the reference list. I was thinking that PNAS is peer reviewed...

All right then, any feedback is much appreciated.
They used MRI to compare asymmetry in hemispheric volumes across the groups. Hemispheric volume is not something that is changing over time, and so it doesn't require the use of functional neuroimaging (e.g., fMRI).  

Instead, if we want to be accurate when measuring volume and making comparisons across groups, we need to have high spatial resolution of the cerebral hemispheres. fMRI does not have the same degree of spatial resolution as MRI. Consequently, MRI is much better for studying anatomy, whereas fMRI is good for studying function (due to its relatively high temporal resolution).  So when imaging hemispheric volumes, MRI is better suited for the task.