So here's the deal, ALL pet foods (no matter how much one person talks them up as being the worlds best) have nutritional gaps. And each food has it's own gap, so it's not a bad idea to switch them every once in a while to fill in where the previous lacked.

I agree bag to bag to bag, is not the way to go about doing that.

I switch mine 3 or 4 times a year between several high foods.

So I have to ask, does she think Iams is a quality food? Is she looking for healthy foods? convience? price? Dig, why does she switch... is it just because 'variety is the spice of life'? Does she realise that quote really only applies to humans?

if health is a the issue, rather than morals, try introducing this site as an educational tool:
http://home.comcast.net/~mstraus/dogfeeding.html