I honestly believe that the forums fulfill quite different purposes.
H&S is a fundamental part of the core business as in the GG business model it represents the 1st line of support.
This is not just important in providing a quick fix to those with a problem, its main value I believe is in attracting new customers due to the consistently high ratings GG achieves in the various satisfaction surveys - Which, MSE, USwitch etc. - and I do believe that this influences people's initial choice of provider or gives confidence in the choice they make.
The GD forum does help provide exposure in the search engines so plays its part, of that there is no doubt, and indeed supports the community ethos which is also part of the GG business model.
If Lithium were to stop working overnight (every possibility) then GG would have to put something in to replace H&S as a matter of urgency but GD would not be such a problem.
That said surely both should compliment each-other, and true although I've never visited GD there have been various negative threads about GD - about 1 every 6 months, raising concerns about the content and conduct of the forum, I've no idea if these comments are valid or not.
So there has been real efforts recently to improve H&S which is good, perhaps if there are areas in GD which need to be improved to make it a more attractive place for all to visit and contribute then this would certainly be a valuable investment of time by GG staff.