A pixie is an epitome of one aspect of fey - the cute, mischevious kind, more than anything else. But 4E has also focused on other aspects - such as the otherworldly, inhumanly beautiful domains of the high elves (eladrin), or their nightmarish reflections of the darker aspects of folk lore, in the form of fomorians, spriggans and such - or even just the cruel members of the Winter Court.
And it is those aspects, honestly, that have more need for producing stats and background for. Thus, there are plenty such fey in the monster manual, and there are write-ups of fey realms like Mithrendain and adventures set there in the online magazines. We have an umbral sprite swarm to demonstrate exactly how vicious a flock of evil sprites can be, in fact.
Sprites certainly have many more characteristics than the common housecat, and are more interesting than one in general. But how vital are combat stats? I'm not saying they never need a write-up - but can it wait for the Feywild book, or the MM3, or another resource?
I remember an early LFR adventure, Lost Temple of the Fey Gods, which has a nice skill challenge used for interacting with a group of pixies. Getting ahold of that adventure may give some decent guidance on how to portray such interaction, since I assume the goal is to amuse the players and bedevil the characters, rather than focus on armed conflicts with some faeries.
