It's my understanding that En Sab absorbs powers when he transfers to another person's body. It's also my understanding that when Prof. X was connected to Cerebro, En Sab was amazed at his extraordinary psychic powers and wanted them.
First question: did En Sab make a mistake, as the psychic powers he detected were only possible through the assistance of Cerebro? So in a sense, he wanted Cerebro, not the Professor's powers?
Also, what happened in the scene where Prof. X enters En Sab's mind? Initially he overloads En Sab with all the voices he hears, but then En Sab grows huge and throws the Professor around like a rag doll. It would appear En Sab has superior mental powers than the Professor, as he won the fight with ease (though strangely it didn't start off that way). Since En Sab has stronger mental powers, why does he want the Professors?
One last part. In the scene where En Sab forces Prof. X to send out the message, can Prof X. normally contact all mutants mentally, simultaneously? If yes, what's the point of Cerebro?