Of the current GN and GA services, the vast majority of the off-peak Cambridge-London traffic is carried on the 2 tph GN non-stops - the GN semi-fast and slow trains, and all the GA trains cater mostly for the intermediate traffic. Plus of course Cambridge is an important interchange with services to/from the rest of East Anglia.
As an illustration, on Saturday morning I travelled to London - I was in time to catch the 09:47 non-stop to KGX, but the 12-car train was packed full and standing so (as I wasn't in a hurry, and didn't fancy standing all the way) I opted for the lightly loaded - when it left Cambridge - GA 10:04 (semi-) fast to LST instead...