re: I can only see one reason
"providing larger labs like the MSL cannot be delivered by the ballistic-bounce method used in MER and Pathfinder. Is this the case?"
Yes,
http://www.planetary.org/radio/show/00000180/
Airbags can't do bigger than MER.
MSL will be actively guided during entry in the same way the Apollo and Gemini capsules were (offset c-o-g) to take out as much of the upper atmospheric uncertainty as possible. The decent stage which will cover the last km or so will also navigate toward the nearest safe spot using radar. Because of the guided entry, the MSL entry ellipse will be considerably smaller than that of MER.
Doug