I believe that is the primary reason - as the branch was trialled with 2x 153s but the local authorites opposed it as there as several schools along the route, for which the 150 is very handy for...So as a result, LM Have 1 service operating with a 153, the other is operated by the 150. Also, current platform lengths do not allow the use of the LONGER (So much for the Go anywhere sprinter replacement as promised!) 23m Class 172s.
So, 1 x 150 operates the Marston Vale services and is outbased at Bletchley during the week, 1 150 operates a service to Hereford to keep up traction knowledge (although i can't remember how many times a week this happends) and the third is kept as a spare.
As for the Exiled Dogbox twins - It's abit of an odd case, as why should they not want to repaint 2 153s with a year left on the franchise, but be happy to repaint and refresh the ex LM 150s? Not sure what work actually took place on 325 whilst it was up at Wolverton, as it was supposed to be receiving a repaint and refurb as part of it's overhaul - Instead, it seems to have back slightly less reliable...