^ how is it possible to treat wives equally?
even phop muhammad (pbuh) 's fav wife was ayesha.
That may be true, but AFAIK even if he wanted to spend more time with Ayesha, he used to ask permission from his other wives first. Otherwise normally his days were divided among his wives equally to the best of my knowledge.
What are you suggesting ?? that its ok for a guy to simply abandon his one of this wives for another one ? NO if a guy is meant to marry more than one woman, he HAS to keep a balance.
Bye the way, if you wanna know why multiple marriages are allowed in Islam, Dr Zakir Naik gives an amazing explanation (youtube).