It's perfectly ok to commute 5 days per week and then do a training ride on the weekends. You need to vary your intensity during your commuting. The secret to riding everyday and not burning out is not to ride at a maximum intensity every single day. You need to have some easy days in between your hard days...Yes commuting can be a great training ride if you push hard enough and especially if you include some hills and sprints during your commute... Once in a while it's a good idea to take one day off and just rest.