I'm expecting that SpaceX will have lots of towers, not just one (currently) or 3 or 4 (under way). It won't just be for redundancy. The duty cycle of a tower might simply not allow for the cadence that SpaceX wants to maintain. With Falcon 9 they currently have a 3x weekly launch cadence (which is unbelievable enough). With full reusability they might be able to get to daily and better cadence, so if the duty cycle of one tower does not support that (I assume right now it does not) then they'll need more towers.