Totally agree with this! There should be a variety of ways to level up - dungeons, quests, defense/invasion, leves, straight up grinding, and whatever else the dev team (or community) can dream up. Not only does having a variety of options help mask the grind, since you can switch to another method of leveling up when you get tired of one way, but it will also help appeal to a wider range of players than if there were only one method of progression. Ultimately, there's no reason why there can't (or shouldn't) be multiple viable group-based ways to advance.
In short: multiple options are good!