It could be argued that that means, if pathable, that a Bletchley stop in the Avanti Birmingham semi fast may be a good thing to add. May not be pathable though. The WMT service to Euston is fast, but that to Birmingham is quite slow, and the fairly sloppy punctuality that seems to pervade these days means a connection from the xx32 or xx39 onto the semi fast Birmingham at MKC is not always reliable. Obviously those from the Oxford/Winslow side can change at MKC, but they have a direct Birmingham anyway, and much as XC is grim I doubt it's grim enough to add an extra change into the journey.
It will be interesting to see if there is much of a London commuter market from Winslow changing at Bletchley, or if Winslowites (!) will continue to drive to Aylesbury, Vale Parkway, Bletchley or MKC as they have done previously. I suspect a good proportion will be the latter as connections are usually not popular. I reckon you may get extra Oxford commuting from Buckingham though (driving to Winslow, or potentially even cycling there as there is a segregated cycle provision all the way, albeit one that occasionally requires some gardening).