Forum:MASS HYPNOSIS

I WAS AT THE LOCAL MC DONALDS THIS PAST SUNDAY and my friend mentioned a duel he and I were playing in which I activated my "Mass Hypnosis" to take control of his face up monsters. for some reason he thinks that during the end phase I would still keep the burgled monsters even though my "MAss Hypnosis" is destroyed during that end phase. the card which allowed me to gain the effect of taking his monsters is no longer on the field and thus its effect would no longer be applied is what I say. why else would this card destroy itself during the same end-phase of the turn it was activated?? why, I ask? to balance it out and keep it from being "broken". what say you?