As for my initial stance on HP scaling. I have indeed given it more thought. And while I still see no reason for it's current implementation, in certain circumstances it would in fact be welcome.

Assuming all other fights were normal (without HP scaling), wouldn't it be cool for there to be one powerful boss who increased in power as more players fought it? And, since it was only a single fight, it would be acceptable for the drops from that boss to scale up with players as well. This would create a challenge: Bring as many people to the fight as possible and still be able to win.

Boom: Difficult content that encourages people to play together.

Furthermore, this would be quite similar to the current Ambuscade system. You can choose to solo a very easy fight for a lower reward, or you can bring a party on a higher difficulty for a larger reward.

That brings up my most important point with the problem: Choice.

For Ambuscade and Merit fights as well you choose how difficult (and thereby what rewards) you get to face. This system includes everyone, people who want an easy and fun game, and people who want more of a challenge. Without that choice all players are forced into choosing the same strategy: Bring the absolute minimum number of players possible to win.

And quite obviously, when you choose the lowest number of people that means many others are excluded.

One more thing: You say players should not have to force difficulty on themselves. But they already do this. When you solo a Merit fight on D or VD, you are forcing unneeded difficulty. When you clear (normal non-HP scaled content) with an unusually low number of players, you are forcing unneeded difficulty. People do this because it makes them feel good that they were able to go above and beyond expectations. But when you know that the difficulty is automatically adjusted as people are added and removed, it looses the appeal. Yay, you cleared a tough boss with only three people, but oh wait, that same boss was actually weaker then it would have been with more people, less yay...