I just don't think the Twins decision makers can be that stupid to rest him/DH only without some justification. There must be some injury(s) or ailments guiding the philosophy, or just flat out his past track record.
Twins are winning, in 1st place and backups/call-ups are proving worthy in the short term. Seems odd to be criticizing it right now. If Buxton can get fully healthy and demonstrate he is handling the workload, then by all means they should ramp up his time in the lineup playing CF.
If they were being forced to rely on the likes if Jake Cave to pick up the slack, I would reevaluate my opinion.