Plusbus is not available within zones 1-6 (apart from Elstree & Borehamwood where a significant number of buses are not TfL). If you wish to avail yourself of bus travel to the station where you travel to London then I agree that maybe a traditional ticket might be better. It’s all a matter of choice and someone who drives or walks to the train station may find that the slightly cheaper daily cap compared to a day travelcard makes contactless more worthwhile.