Thanks and great question.

I did not go into detail on this point, but the way I see it, the purpose of Green-tagging is to sort of highlight the most relevant few of an author's original tags. So all the original tags are still there, but curators decide which ones they think are most relevant. For some articles, that might be just one tag that almost everyone will green-tag. Remember that the green-tagging influences rank based on a custom per-user filter (if they chose to set it up) on SteemIt. It doesn't cancel out or overpower the original tags from the author, no matter how many green-tags some tag gets, the article should still be searchable using the original tags...

For some articles, like in your pokemon example, it might actually pay more to green-tag three different tags: since other curators are likely going to feel these are the most relevant too, and your curation reward is split between the 3 but ends up being greater overall, thanks to high numbers of green-tagging on all three tags you picked.

I think we have to limit the number of green-tags because if we don't, then bots will blindly upvote everything on everything, in an effort to grab curation rewards unfairly.