Your point is valid and that is why the trans ALT had been designed for the approach page in the first place.
Now, by an one Airbus TRE it had been suggested to me
that this function is best utilized in another way - as a transition LEVEL reminder. The logic is that should you forget to reset altimeters after GA, you create a level bust. The very same applies for not changing to QNH/QFE. But, how many GAs you experience per year as opposed to how many approaches you execute?
Yet I do not do it, because my Airbus teaching leads me to believe that it is best to use the features the way they had been designed. On some occasions, I decide to apply the above stated reasoning and use it for reminder. Once
it did help.
FD (the un-real)