Letizia loves to wear them- she has probably worn the most of any of the royal ladies.
She had a lacy one while she was pregnant with Infanta Sofia, a floaty chiffon(?) one that she wore to a party just before/at the very start of her last pregnancy (she had her hair down and curly, if you look for the picture), a nude colored sparkly one, a black lace/white sateen dress (non-maternity), plus some 'suit-dresses' that she has worn- like the pink one she wore to Beltrán Gomez-Acebo and Laura Ponte's wedding.
I'll have to see if I can find the pictures.
Found some- there were more than I expected. Some of Letizia's cocktail dresses border on day dresses or suits, but then again, that's just Letizia....I included the occasion she wore it, and the designer, if I found it.
Cocktail Dresses 1
Dresses, left to right, top row: White dress with silver accents on the bust, a farewell reception in Palma, by Varela; black lace over light underlay, gala of CIERVAL; Gathered and ruched pale crepe (i'm guessing) with accented banding on the bodice, at a wedding of one of Felipe's friends, possibly by Varela; Black or navy lace from a dinner for the Hungarian president, by Varela
L to R, bottom row: Pink satin dress with lacy (or embroidered sheer) overlay, Mariano Cava's Journalistic Awards; Sparkly gold/beige dress, Laureus gala dinner by Varela; Antonio Asensio Awards, by Varela (same dress as the previous, but with a coat); Black dress with lace over-blouse, Awards Ceremony of the Chamber of Trade of Terrassa, by Varela (looks different from the one above it- the underdress is certainly not the same).
Cocktail Dresses 2
Top row, left to right: white dress, one of her first maternity dresses when she was pregnant with Infanta Leonor, in Japan; silver suit outfit (there's something Jackie O. about it- I like this one) at the wedding of her cousin Abigaíl Rocasolano, by Miguel Palacio ; Ruffle-y pink skirt dress with matching pink shoes and jacket, at the wedding of Beltrán Gomez-Acebo (Felipe's cousin) and Laura Ponte, by Lorenzo Caprile; Little black dress with silk or satin pale pink jacket, from the May before Leonor was born; Coral dress with beige coat just after the announcement of her pregnancy with Infanta Sofia at the Spanish National Day Military Parade.
Bottom row, L to R: Black and white dress with large pattern and black coat while expecting Infanta Sofia, Spanish Geographic Society 2006 International Prizes; Purple (or maybe royal blue) dress, early when expecting Infanta Sofia; Pale pink dress with cardigan while expecting Infanta Leonor, during a visit to Ciudad Real; Red dress worn in Japan, it was one of her first maternity dresses when expecting Infanta Leonor (thankfully they got better after that!).