Eleven units out of twenty three would be a pretty poor fleet availabilty though, when a typical DMU fleet can expect to acheive 80% utilisation or perhaps slightly more. It's worth bearing in mind as well that some of the diagrams on the Hereford and, I think, the Shrewsbury lines are formed of pairs of 170/5s and there's also bits of additional peak time strengthening goes on.
Nonetheless, your point still stands that London Midland seem to have quite a low utilisation of their 170s, and it seems strange that the DfT would assign stock away from LM, in the form of the three 153s that have gone to Northern and FGW (That is right isn't it? One at Northern and two at FGW, as the two that were at NXEA no longer are? I lose track) to the extent that it would impact on capacity as far as to cause overcrowding.
I had hoped that LMs' loss of some 153s would have seen some re-arrangement of 170 diagrams, so that pairs of 170/5s would have replaced three-car 170/6s on the Hereford/Shrewsbury services (where four car trains are most needed on busy services), allowing more 170/6s to operate the Chase line, making up for the lost 153s. Sadly, this has not proved to be the case. As you say, it probably boils down to that number one limiting factor: That of cost.
Although that said, if LM still have seven of their once ten-strong fleet of 153s, then to my knowledge there's only currently a requirement for four of them (One on each of the Chase, Snow Hill, Cov-Nuneaton and Bletchley: I think the Hereford diagram has stopped), then surely there's a little bit more availability to be squeezed out of the fleet to give the Chase line an additional 170/5+153 formation?