Hope this doesn't come across as harsh Sam, but if you haven't experienced the problems with a 88" (or shorter) offroad then you haven't been driving very difficult terrain (or at least not steep+muddy).
A few examples: I have seen/experienced several SWB landies which have spun around 90o or 180o while going down steep (muddy) slopes. Most recently, a member of GCLRO at an outing at Levuka needed to have a strap connected from his S3 SWB to a landie travelling behind him so he could go down slopes safely without spinning around. The slopes were not especially steep, and the S3 had mud terrains and a rear locker.
The most serious incident - Ian (forget his last name) from LROCB many years back in a twin-locked soft top SIIA almost rolled backwards on a steep slope. Luckily he was on the base of a hill so was saved from rolling back completely by his aftermarket spare wheel carrier. (there is video floating round of this incident - unfortunately in VHS format only).
There are also videos floating round on youtube of an 80" ibex coiller rolling over forwards while going down a slope that would be no problem for a 109".
The 5" longer wheelbase of a 90" defender seems to make a big difference, however almost all serious/competition 4x4s are 100" or longer in wheelbase.
I agree that in standard form, a 109" is generally severely restricted by stiff springs, whereas an 88" has quite supple springs as standard.
EDIT: This is one of the problems I am talking about - have a look at 0:14
Video Link: http://www.youtube.com/watch?v=VT1f6JN9BRY
An 88" wheelbase on the same track would probably have flipped.
EDIT2: Here is the ibex vid I mentioned - 80" WB I believe.
Video Link: http://www.youtube.com/watch?v=B4p9pB91vUA
This hill would have posed no problem for a LWB.