The chassis can still flex and change the mesh. Plus the added weight of the clutch will wear the bearings out faster, no way around that. They should incorporate a front bearing block to support the end of the clutch assy. Sure it would add some weight, but if you are hell bent on having a clutch you are not all that worried about weight...
+1. Having the pinion and spur on the same mounting plane makes chassis flex irrelevant. Any other mounting scheme has the possibility of slightly adjusted mesh when the chassis flexes (and it does). The best way I've found to eliminate forward/backward flex is to make a top plate that attached from the rear diff, center diff, and front diff.