I noticed in Rail magazine that 14 class 66 locomotives are stored out of use. This is not many out of a class of several hundred but it illustrates a more general point. Rolling stock has such a long life, several decades, and in that period changes in economic circumstances mean that it may spend extended periods under-used or out of use altogether.
There is a good case for procuring units of lower initial cost even if the running costs may be higher. This is also why it may be desirable to retain old stock in storage rather than rushing to send it for scrap.
On the passenger side, it is a good argument for using locomotive-hauled vehicles rather than multiple-unit trains, which gives the operator the flexibility of using a variety of different power units depending on what is available. This is why locomotive-hauled passenger trains persist despite all attempts to get rid of them. However, it is of course desirable to run in push-pull mode where possible to avoid the need for locomotives to run round trains at terminals, although it would be no bad thing if facilities were more widely available to allow for this. It is unfortunate that these run-round loops were taken out where they were already present - it shows a lack of forward thinking.