would you spend 1$, to mail in a coupon, for 1 penny off a online purchase? Sure it makes that thing online 1 penny cheaper but you just lost 1$ doing it! Who would do that? This is what the change prevents.
if it cost the sender more than the amount sent, why would they do it indeed...
they would only send it if it were in their best interest, as determined by them.
someone might not spend $1 to send a penny coupon, but they might spend $1 to to do any of they many things you can do with a penny. sell it to a collector, melt it down for copper, put it on a railroad track, squash it with a commemorative logo at Disney land...
there are even more things you can do with just 1 satoshi.
there are pros and cons to dust, but there are also unforeseen consequences. just be aware of that.