This mechanic is very detrimental to the community.

Firstly, it makes it very difficult for new players to make progress with their equipment, since any character that cannot 'pull their own weight' in parties makes it even more difficult to complete then it would without this mechanic. This discourages established groups from accepting new members, especially those who are already having some difficulty clearing content, thus meaning that new players cannot help other new players and are forced to rely on veterans who have already completed most of the content in the game (who have little incentive to do it over again just to help new/undergeared players).

Second, when combined with timed fights, this forces the community to use only the optimal job setups for each fight. This is a major contributor to our current state of having less then half the jobs available in the game to be actually used. If parties were able to bring more people to endgame content there would be more leeway available for people to fill with jobs that they actually enjoy playing, rather then being forced into cookie-cutter setups with only the absolute minimum number of people required. This is supposed to be a massively multiplayer game, and while I know the population is declining, it should still encourage people to play together, not exclude others because bringing them along hinders their chances of success.

Mechanics like HP scaling are quite obviously damaging to the longevity, and more importantly the enjoyment of the game. There is no reason that it should be included, anyone who clamors for more difficult content should find their own ways to make the game more difficult (such as bringing sub-optimal jobs to difficult content). Intentionally making it more difficult for people to play together in a multiplayer game is simply bad design. I understand that the current team of developers is not familiar with some of the older mechanics and may have difficulty alleviating them (such as the exp penalty for larger parties that was included in the original release), but this is a relatively recent mechanic and should be easily removed, please do so as soon as possible.