Is there any way that we could have Hidden Dimensions still influence the aggregation behaviour?
Currently, when you hide a dimension, only the visible one is taken into account during aggregation.
It'd make a whole lotta more sense if they WERE taken into account - after all, if you took the time to create a dimension and hide it, it's because it's needed, not because you did it just for the fun of it. ;-) Otherwise you'd just not add the dimension to be hidden in the first place.
Below is the best example. I have a Google Sheet with Date & Currency Code as dimensions + 1 exchange rate metric only. Pretty simple. I then create a custom dimension and bring in Currency Code from all platforms I need (GA, AdWords). It works GREAT, except when you hide the column... (this also happens if you add a total column). The correct value is 152k not 146k. 146k is taking the average exchange rate for all currencies & using that instead, which is, financially speaking, absolutely mental!