Well that is the simple face of it sorry
We only ever see four of them working the Chase Line on any given day, two 170/5 working on their own, 1 170/5 with a 153 coupled to it and a 170/6
The Shrewsbury runs are taken by either 3 x 170/5 or 2 x 170/5 and a 170/6
The Hereford and Great Malvern runs are taken by 3 x 170/5 (please note that I don't know this one for a fact because I myself have never been down that way but am relying on my brother's information and sometimes, he is useless!)
Let's have a look at the other routes that LM have:
The Crewe - Euston service is run by 350/1 or 350/2 desiros
The Birmingham shuttle to International is run by 323s
The Birmingham - Coventry, Northampton or Euston service is run by 350/1 or 350/2 but mainly 350/2
The Birmingham - Liverpool runs are run by 350/1 or 350/2 desiros
The Euston - Tring, Milton Keynes Central or Northampton is run by the desiros but sometimes they might stick a 321 on the service to Northampton
The Watford Junction - St Albans service is run by the 321s
Cross City Line is run by the 323s
Coventry - Nuneaton is run by the 153s
Bletchley - Bedford is either a 153 or a 150
The Snow Hill lines are run by the 150s soon to be the 172s
Other than that, there just isn't anywhere to stick the remaining 170s unless they apply for more local routes in the midlands but I don't think there is any left!