It seems there is a reasonable amount of overhead (and some risk) for participating in the baking process on the 48 hour test chain during the voting period.
As far as I can tell there is no incentive for a baker to actually participate in the test chain baking outside of simply "it is good for the network", and there is also no downside, so long as a critical number of bakers participate.
I guess I see it as similar to the choice of whether to run the accuser or not, for an individual baker there is little need or incentive.