Every Princess travels representing their countries, none of them are less than Letizia, other's even travels for business much more as they have their own agendas, she's not better not worse than the other ones who still managed to do their "business trips" on a more fashionable or royalty way and I don't see them as a fashion models as for Letizia leaving to them the "fashion plate" as you said, others dress in a more distinguish manner (and yes, more fashionable) take Mathilde or Maxima when on meetings or delegations abroad (not talking here about Opera or Theather or Gala Dinners since Maxima is a little bit "too much" in that chapter), but when in business trips they go decents, impeccables, fashionables still not boring with a good cut clothes and good fabrics, accesories etc; and without stopping being princess and represent very decently their countries, she's above all the Princes of Asturias, the royalty is also about glamour and fashion as silly as may sound, the fact that someone dress poorly doesn't means they job is better or viceversa; so I'll rather will take the time to mix and match some of my best clothes because I'm going to a country for the first time as a Princess, representing Spain and the Royal Family; she's not the only princess doing her job here, other's have meetings even w/ the UN by themselves, and they managed to go in a "more royal" way; Letizia is not a corporate executive not a politician (and even they dress better) but the Princess of Asturias. Just my opinion.
BTW. For example, I can picture her leaving the plane at the arrival with a suit like the one she wore on the engagement day (the Armani one), and I don't mean it has to be an Armani but the same kind of cut and fabric, withouth a shirt inside, in a pastel color (peach, blue, even red), with a nice pin with matching earrings, a nice hairdo (to avoid the wind and the hair on her face) and a pair of closed pumps and a matching purse, she has beautiful matching sets on that department, not with the kind of sandals/espadrilles she used that day, nothing fancy, nothing to break the bank about, but with much more class and definitely much more elegant, in case someone may think I want her to dress on expensive designers only. That's only one example as she can be really well dress, in a kind of "executive" way but still as a Princess.