Otsuka has an ipo to generate revenue for the company. As his letter states the revenue will be to increase Otsuka presence worldwide, amongst other things.
From a business standpoint buying out another pharma company would make sense.
Currently, the Japanese have been in a buying mood with Astellas, Sunovion buying Seprocor, Diaynkp Sankyo (Millenium) to name a few.