I understand what Toots is saying, though. I like CluelessBride's take on things. But for better or worse (no pun intended), there are families and communities in which this is so ingrained that even if everyone is perfectly polite about it, it will seem strange, to themselves and to others, if guests who are able to don't give cash gifts at least as large as a reaonable estimate of the cost of their "plate."
That doesn't mean that guests have to do it or that couples can demand it. It doesn't mean that etiquette requires it. The closest analogy that comes to mind is how giving a "thing" gift in a community where cash is the near-universal gift isn't a violation of etiquette but nevertheless seems unusual to people.
It just means that it is unrealistic to pretend that it won't feel to the givers, recipients, or anyone else like a deviation from the norm. Of course it would still be rude for anyone to complain or to criticize. But you can't stop people from feeling what they feel in terms of appropriateness, cheapness, and generosity, and those feelings will be based on what they are used to doing and seeing. And many givers want -- even though etiquette doesn't require it - to feel like they are doing what is customary, or want to give a generous gift in light of the recipients' community, even if it isn't their own. I think that is why Toots was stressing that this plays out from the givers' end, not the recipients'. Is that right?