Board Thread:General Discussion/@comment-31283382-20131101162239/@comment-1268838-20131104141637

Aisslinger and Demoura were both Arrancar that had no Zanpakuto. Strictly speaking, they were still more like Shinigami than ordinary Hollows. The implication was that they were simply too weak to retain a humanoid form (seal their power into a sword), though Aisslinger suggested that it was intentional. I somehow doubt that, though.

One thing to remember is that strictly speaking, most Hollows never even become Adjuchas, let alone Vasto Lordes. Risking devolution to a Gillian is a far worst fate than just counting their losses and becoming an Arrancar.