Hi Chris.
I think you are right about reliability and cooling both being equally useless.
However, at the back of my mind from a couple of years ago I recall Joey McLane and Marcel Smid used to prioritise development of cooling over reliability (after everything else was maxed out) because according to the in-game tip, cooling reduces engine wear during the race. We all know worn engines are slower which is why in a competitive elite tier you need to renew them every race. For this reason, if I have a choice between CDs, one with cooling weakness the other with reliability I always employ the latter. I have no data to back this up, it's personal preference.
In his advanced guide Joey says:
A good (but expensive) way to improve your weakness if you cannot find a good CD is to get 2 CDs with different weaknesses and to swap them mid season.
When Joey recommends different weaknesses it is in the event you can't find any good CDs. If you find one that it strong in Acc but weak in an important area such as Braking, you can compliment this with a CD that is strong in Braking and weak in say fuel economy. The second CD cancels out the Braking weakness of the first and you only take a small hit in FE.
Also, this was before the "Three CD" strategy became widespread amongst top managers.