Not likely really. There are the technical reasons given by others on developer effort to integrate them properly without breaking other things. But there's also the ToS. They would lose the leeway they have for addon usage to be a grey area. Right now, they can point to the ToS when there is a conflict. And turn a blind eye when there isn't like when addons are used responsibly. They chip away at that leeway if they started allowing them. If it's a useful enough feature, it can be considered for full integration, though that gets into the technical development requirements and questions of available resources.