Hol and Charlie -
Consolidations were used as switchers on many roads, including the Burlington. Would the removal of the pilot truck on a D-4 have made that much difference? The G-10's converted from the Q's heavy prairies (where removal of the pilot and trailing trucks and reduction of driver size really did matter), developed more tractive effort than the D-4's and had a smaller wheel base. Why wouldn't they be preferred as switchers?
The Burlington seems to have used 8-wheel switchers only where traffic was particularly heavy. All the Q's 0-8-0's — the F-2s, the USRA F-1's and the sole F-3 (5020, converted from an O-1) — were assigned to areas of heavy traffic: Chicago, Galesburg, Centralia. Elsewhere, 6-wheel switchers probably were sufficient.
I'd assume the conversion of the T-1's to F-2's was to produce a very heavy switcher to handle long cuts of heavy cars. Those things developed 60,700 lbs. tractive effort, substantially more than the D-4's did or would have had they been converted to 8-wheel switchers.
Jonathan