Porterbrook charge significantly different rates (more expensive IIRC) for their lessors to use the 350/2s, and as I understand it, WMT wanted a Porterbrook-free fleet on the WCML. (They won't have a Porterbrook-free fleet throughout, as the gangwayed Class 172s are PB-owned, but that's beside the point).
The exchange of Desiros for Aventras also seems to allow for more 8-car and 12-car 350 operation, as 37 4-car 350/2s (148 vehicles) will be exchanged for 29 5-car 730/1s (145 veh's) and 16 5-car 730/2s (80 veh's); a net gain of 77 coaches. However, financing is due to be led by Infracapital & Deutsche Asset Management (i.e. not Porterbrook).
Despite the opinions of many on this forum about Abellio, I believe they've got a good set of cards to play and are playing the long game; however, this is getting off-topic!