with Chaos you are going to have a reletively low model count anyway, and as such any way of getting your units into a position to do what they do best that bit quicker is something to capitalise on. As HV says, they are a damned site cheaper now and so are more tempting.
traditionally i have always gone mech where possible with chaos for the reasons stated, and i certainly do with the sisters! in the main it has worked for the better.
another nasty trick with enough rhinos is to block off an area of the board so that you make your opponent go where
you want him to go, so even if it is just for that the rhino can be very useful