The only reason why I fold the buggy is because I can do it when I am travelling with ONE toddler and my children are not keen on being strapped unless tired/asleep. So far I have avoided using busses on my own when I was in charge of both of my toddlers. If I was forced due to circumstance to use busses with them on a regular basis, I would most likely turn into inconsiderate traveller who is banging on about me and my children's rights.
I admire Roz for using busses on regular basis with her children, something I am sure she does not do for her entertainment. Her early comments on this tread were very descriptive of what challenges parents face, and she is blamed for putting her children in certain situation ahead of someone who has not yet boarded the bus. Three lives of her children (and children are emotive to its parents the most) are what she protects.
I've seen many parents miss the busses and I see no wrong in trying to explain that we are all human, but somehow if with buggy we face extra animosity during our whereabouts. Our motives are questioned for the choices we make, without listening to our explanations.
And my hypothetical woman with an attitude and oversized buggy and shopping on the buggy, may need herself help. She may appear rude, because she is tired due to missing few busses, disturbed sleep, or she is adjusting to becoming a parent and has many worries on her mind. You will not bring a positive reaction out of her by criticising her, as she may understand your stance as a threat and would most likely fight for her child's rights to be on the bus. You may not be able to solve all her problems but she also deserves tolerance.