Some back of an envelope math: a smallish arm (under a meter long), able to reach to the ground a bit ahead of the body (in view of the mast cameras) = quite a torque on the suspension differential when at full extension.
In searching for something to offset the weight at the rear of the body, the first and obvious choice would be the batteries. It's unlikely I'd need to operate in extreme cold (below -45C) so placing them outside the body at the rear, and at an angle down into the body for adequate departure-angle clearance, seemed good.
Of course when I pictured this in my head I had an 'ah-ha' moment:
I'm sure there were lots of considerations for placement of the MMRTG's, but offsetting weight seems like one of them.
MSL overview: http://spacecraft.ssl.umd.edu/academics/791S14/791S14L09.EDL2.pdf
Arm doc: http://www.esmats.eu/esmatspapers/pastpapers/pdfs/2011/billing.pdf
MMRTG doc: http://mars.jpl.nasa.gov/msl//files/mep/MMRTG_Jan2008.pdf