The Shell Shock ability is 45 munitions.
Point blank engine shot is 60 munitions.
It's range of 5 is 3 times smaller than an at nade which is more than 2 times less expensive at 25 munitions and doesn't have a high risk of losing the unit associated with it. The 6 seconds long Shell Shock is not nearly long enough to guarantee an escape (even if we ignore that the stuart will target infantry first when Shell Shock is used, which causes a loss of already precious time). In order to try and save the stuart and get an engine shot off one will have to spend 105 munitions or the equivalent of 2 teller mines (or a little less then 2 m20 mines) or 4 at rifle grenades all of which have a higher chance to succeed and cause more damage without taking losses as valuable as a stuart.
Topped off with a reasonable chance of it not even working because, as with all tanks when near other tanks in coh2, the stuarts driver may well lose his shit and start pointlessly rotating.