Well, not only that you can see it more easily, you can also click on it more easily. Let's say, there's an area covered with lots of jellopies and sticky mucus, and in the middle, there's 1 rare card, take two players, one with the modification, the other one without it. Who would be the one to loot the card? Clearly the one with the modification, since the other one probably could not even see the card, thus, he couldn't even click on it.
That's what you would consider as unfair, although you're right, it's probably only relevant in cases of loot stealing, since loot dropped from a monster that you killed is your loot, and if you pick up everything, and drop the worthless items afterwards, it's the same as if you only have to click on the card, except for maybe a time difference of 5-7 seconds.
Then again, look at other online games such as WoW or Runes of Magic, those have legal add-ons to make you see monster HP and other neat stuff which definitely gives you an advantage over players who do not use such add-ons. I dunno. *shrug*