That doesn't sound very logical to me. Any item of designer clothing is instantly recognizable as an expensive, quality, well-made piece, never mind what label it is.
To those of us with a discernible eye, the provenance of most of the items she wears can be identified. If she wears a Prada jacket to an official event, for example, it is almost immediately identified in this thread. So I don't buy the logo argument because that would imply she's trying to hide what label she's wearing. I don't think that is possible in this day and age when everybody has access to her pictures and what she wears is reported on all over the place.
My point is, those who know designer clothing, will know what she wears. Many, if not most, people she meets during her official engagements, however, won't (no matter what the label).