One thing we tend to forget when the trend is toward lighter things is that for machinery weight is good. The extra mass helps dampen noise and vibration. I forget the technical details but cast iron is especially good at dampening vibration.
I play with old woodworking machines too. When it comes to cast iron on wood and metal working machines it's "The More the Merrier". If you ever get a chance to compare an old cast iron topped table saw to one of the new plastic marvels the difference is obvious. I would expect the same to be true for sewing machines, just on a smaller scale.
Rodney