Let's say having a 16 starting makes me happy. However, I want to play a race that doesn't have ability bumps to provide that. I had two options:
- Get a 16, but play a race I don't want to play.
- Play the race I want, but start out with less than a 16.
To some, this is a fair trade off; you have to choose one or the other. But either way, I am in net not happy because I had to compromise something about my character.
But let's just say now I have the third choice to assign my scores. I get both my race and a 16. I am happy. What are the downsides?
Am I more powerful than either of the two options above? I don't think so, outside of some niche corner case somewhere.
Am I ruining the race for others? No, since others can use the original mods without change.
Like customizing my pizza, I can choose to make mine pepperoni, bacon, mushrooms and olives instead of choosing from either meat lovers or vegetarian.
The only people who are unhappy about this are the ones who feel I should be forced to compromise one or the other of my happiness for reasons. The ones who only want you to pick off the prebuilt menu and not be able to customize my pizza toppings.
Life is too short to compromise on pizza toppings and D&D.