In fact, the main issue is that when someone draws a new roundabout or update one, since the oneway is not there becuase implied, it will not alwys show arrows, and I have seen several roundabouts that were not in the correct direction. Having the tag oneway=yes would have shown the error right away to the user. I understand that implied values are nice to reduce the number of tags, but in that case, it could result in erroneous data because of the lack of feedback. As a rule of thumb, I think that we should never remove implied tags that someone added, because the "duplicate" info here could help confirm the direction and make sure the arrow icons are shown in evey renderer. Just looking at issues in the OSRM project, I can find some issues mentioning that the router did make them use a roundabout in the incorrect direction, for this specific reason. The roundabout was drawn in reverse order and the lack of feedback made it hard to detect. Just imagine a UK user updating a north american or other european roundabout and, by reflex, they could draw it in the reverse direction (clockwise instead of counter-clockwise). Recent version of the id editor are indeed showing the arrows even when the oneway tag is not there, but other editors may not. It may be that all recent editors are correctly showing the arrows and recent routers may all imply the oneway tag for roundabouts, but I would like that until it is proven that all of them do, keeping the old implied tag is way more secure and robust against routing errors.