I think mostly it's just a constant reminder of the effect Fleur has on the average man - Ron is always a bit of an everyman. The real question is not why Ron is so susceptible to her charms, but why Harry isn't. I'm making up a theory on the spot here, but do you figure it's meant as simply a more extreme version of how men would treat an extraordinarily attractive woman in real life - and that Harry's reduced susceptibility to her is because he actually knows her a bit and doesn't just think of her as beautiful?

Of course, Ron was more susceptible to her than Harry from the start, but then Harry was a bit distracted by his crush on Cho at the time. Both are equally affected by the full-blown Veela's (this is another case of the Dutch translator coming up with brilliant finds, I actually thought the Dutch name was the original: Glamorgana) at the World Cup, though.