If a woman wants to waive her rights then she can do that in a conventional nikah any way. So what is the big deal if she does that before signing on the dotted line and the nikah is known as 'misayr'. It is not as though she is marrying for a limited period. It is not the same as Muta (temporary marriage)e.g a passport for sexual relations only.

Oh come on, don't be so narrow-minded! What makes you think sex-on-tap is the reason for this kind of marriage. How about loneliness? Company? Maybe a woman is independant and doesn't need a man 24/7. What about a business woman who is self-reliant and doesnt need a husband to maintain her yet still wants to experience the joy of a caring and loving relationship?
What is better? She gets married and waives her rights to having the man to support her financially and be there for her 24/7 or to not get married and have a haraam relationship?

Even in a conventional marriage a man can marry again without his wife's consent and should that happen she'd have to adjust to him spending time with the other wife. So if the other woman waived her rights in her marriage contract stating that she'd be happy for her husband to spend only two days per week with her then that would be a masayr marriage contract. So what is wrong with that? The man would still be responsible for everything that the woman hasn't waived her rights for. I think masayr marriage has been blown out of proportion, making it seem like a no-strings-attached affair! It's still a marriage, the only difference being that the woman has by will given up the rights which may not be necessary for her.