Most MMOs have a varied assortment of content. However, not everyone enjoys particular content styles. There are a lot of PvE players who don't like PvP, and vice versa. There are players who only want to do small group content or large group content, but not the other type. There is always an argument among the playerbase on whether it's a good idea to encourage players to participate in content styles they do not enjoy. The most common argument is that it makes the world feel more alive if there's more to do. However, I personally am of the opinion that forcing players to do content that they do not enjoy, for example through exclusive player power rewards or cosmetic rewards, will make them enjoy the rest of the game less and in turn stop playing sooner.
What is the potential game design rationale for an MMO to force PvE players into PvP and vice versa, or to force players who prefer to play in small groups to do large group content?