You sound just like the pigs there. "All animals are equal, but some animals are more equal than others."
This sounds a bit offensive...
To topic now...
From this rule only high tiers are affected. At low tiers (3 and below) there are not many dungeons with high mob population. This means that low tier players still will advance to high tiers "quick". But from tier 4 and above leveling will be hard.
Yesterday i calculated that i'll need 8-9 Aps to take the level... Due to this rule i used 13 Aps...
Also it is a bit weird that 2 rare mobs giving 2300+xp and 1 rare with 4 simple gives 1400xp almost 1000 less.
As you have noticed the rule announcement is gone... This means there will be changes to this rule. So instead of arguing if it is good or bad, lets post our suggestions to help them come out with something better and more balanced.
Here is mine suggestion.
Instead of "punishing" the players for not using merc, you should "award" those they use, the result will be the same but the way there will be nicer...
For example:
Now if you go solo to a "populated" dungeon for leveling you will get, lets say, 2000xp per AP(with the rule applied is 1000).
If you use 1 merc, you will get 1000xp per AP.
But if you reward a player when use mercs, it will be like this:
You go solo you take 2000xp
You use one merc and you get, lets say, 1500xp (instead of 1000xp).
This means that you are awarded with 500xp for using merc. You also use merc's bonuses, fight is over with less hp loss (means less gold for potions), bonus to gold/item drop, etc.
And you give the choice to the player between quick level up with less gold gain.
Or normal level up with merc use, and bigger financial benefits...