i've seen a few of them out in the wild too. makes sense. it's about on par with the moto g for what it offers, yet half the price. i think some of the low buck MVNOs like cricket and metro pcs are actually pushing them in stores. which has to help.
Cricket for a while was offering $50 635s with a $50 rebate after a month of service, essentially making it free, so they sold plenty. The 640 is $99.99 though, so it's not getting such a generous offer. It would actually be cheaper to buy an AT&T 640 from the MS store for $79.99 and pop in a Cricket SIM. Black model only, compared to cyan for Cricket.