I agree with 74ford, the turntable would be a common point of passage for nearly all the locomotives, whether they were going into the house or a storage track. Those toolboxes in the photo of Gunnison are not nearly as big as the wheeled toolboxes in the Durango shops (which were originally owned by individual shop men), yet they would become quite heavy when fully stocked. Note that the outriggers and shelving at Gunnison were built quite stoutly, apparently to hold quite a bit of weight. Also note that the shelves were at a height that would require the least lifting for the men handling them. They were needed on both sides of the turntable because the toolboxes were stored in different places on the various locomotives, and no one wanted to have to carry them around the length of a locomotive to reach the storage area.
As someone else in this thread suggested, storing the toolboxes in plain sight, yet in an area that would not have much foot traffic, might make sense for security. But the area around the ash pit might well be less visible and therefor less secure. Also it can be clearly seen that those boxes had hasps on the lids so that they could be secured with a lock, if desired, to prevent individual tools from "walking away". Whole toolboxes would not be much of a temptation, as carrying them very far (and without being seen) would have been pretty difficult.
Remember too that these toolboxes were intended for emergency "running repairs" out on the road, not shop crew use. So they did not need to be in a convenient location for repairs while the locomotive was in the shop. The shop crews had their own tools for that.
My 2.5 cents.