I would say: It's not a bad Idea, but it's not useful ... Shift-clicking is supposed to makes you spend less time placing items ... do you really spend hours placing upgrades ? I guess not. So it's not bad, but I don't see the interest of it.
I don't spend hours placing upgrades, but I don't spend hours moving armour to my inventory slots, moving humus to an arboretum in Forestry or even organising my inventory. What I do find this useful for is new players who expect shift click to work on machines, move the items into the aforementioned machine and cause spillages from their piping systems. InvTweaks was not developed because people spent hours reorganising their inventories and it does not save that much time. It was developed because it was a small improvement on the previous system that just made things more usable. Look at the king of usability, Apple; While I may not like their products personally, they have nailed the seamless UI with shortcuts and tricks working system wide. Their profits show it too.
Honestly, I would expect this change to be a couple lines if the devs have already implemented the transferStackInSlot method and a quick copy/paste if not:
I didn't mean to start a flame war, so let's all stop shouting at each other over the internet. Honestly, I'm not bothered if a suggestion gets implemented. I understand there is limited time and not every little detail can be attended to. It's there if someone has a spare ten minutes though and there is no negative outcome to implementing this that I can think of.