First, how would adding the use of 'some' mods allow SE to regulate their usage? Currently, nothing is allowed and SE cannot regulate it. It isn't even a simple case of SE going, yes, yes, no, yes to various mods as that would require someone, or most likely, a team, to vet each one and any updates that might come after, to make sure nothing was added after. However, it doesn't stop people from using XIV launcher, which makes the whole thing irrelevant anyway.
Second, people would still try and push the bounds of what is allowed. You say all UI mods are fine, ok, so, I come along with a mod that dynamically changes the look of the map to show you where safe spots are. Technically, I am just altering the UI, which is allowed but it is obviously against the spirit of fair play. This is just an obvious example I quickly thought up, however, you can be sure that people will try and ride that line as close as possible.
And lastly, they have already said that they will be looking at popular mods and consider adding them into the game. The first obvious example is the large cooldown timers on actions. There are some things that will obviously not be added, like a parser, but who knows what they have planned.