Mnk is by far the job I am looking most forward to and the one I believe has the highest potential of any job. Mnk classes have always been my favorite in games so I may be a bit biased, but I decided to look at all dps abilities they will gain closely, and looking at War and Drg I said "That's it?" War get abilities they are almost all for tanking, and drg get a jump and a fire breath. Mnk get to hit faster and kick! As for the kicking costing MP and being an addition ability, I don't mind it, because I don't feel getting kicking standard would have the same Impact. Okay think for a second, how can you replace the kick and still have the same affect? Oh let's add a third punch, hmm okay it's good dps but not that exciting. However with kicks, your Mnk, just aesthetically goes from just punching to adding a third hit, and it's a kick! This will have a serious visual impact and really make you really feel like you are bringing out the beast.
Now the game play side.
Why does it cost MP? There is NO other way to do it. Get a Mrd friend, go to Zahar'ak and do a little test. Have him ramp up Berserk while you just auto attack. Both with Ifrit weapons my Mrd friend hits maybe 300 damage a hit. I forget my exact but for now let's say I do around 130 a punch, it's somewhere around there. So 260 a round, 40 less then him. That makes sense though right? He has this huge axe, and Mrd are supposed to be massive hitters. However one thing people seem to forget is, Pgl/Mnk hits
really fast. I hit almost twice as fast as him. Now, then go over to Grid or LL, and find yourself a crab. Hit it a few times, then pop Fire Fists and watch what happens. Your dmg almost doubles. So paint this little picture as Mnk.
You are a Mnk, hitting fast and doing strong damage. Then you use an ability that makes you hit even
faster, with no cost to yourself. THEN you use an ability that adds ANOTHER hit per round. THEN you use an ability that vastly increases your damage on certain enemies.(keep in mind Mnk have three different elemental affinity stance)
Yes your MP will be draining fast. Probably faster then a Brd can recover it. Maybe a lot faster. But if it didn't drain MP, Mnk by far would be the most OP melee class.
I will enjoy my MP kick attacks. Because I will enjoy going from a standard AA dps, to a massive speed, kicking demon with the power to envoke the elements in all his attacks.

Just the shock of switching between the two will really make you feel the power of the Mnk, and appreciate those kicks so much more.