Dan, it is important that any kind of label be able to hold up over time. From my own past work, I can verify that a shoddy label system is one of the first things to make an installation, an entire institution, however large, seem dated and uninviting. Also, consideration must be given to the environment in which labels are to be located. Many climates, and internal institutional climate systems, can be very hard on how well a label holds up, quite apart from the impact that on-site visitors may have on them from simply touching a label (which is done so-often). Even the hardiest label often has to be re-placed in order to 'keep up' with prevailing conditions and desired appearances. 
No doubt this topic has many other considerations, and the institution's budget is one of them (not only for the initial creation of the system, but its maintenance and replacement). And there may be other considerations.
Although the 'identity' of an item (in the broadest sense) may seem fixed, additional information is often developed with time, with new registration methods, with updated donation acknowledgements, and/or with new forms of institutional analysis. A more up-to-date label can be desired/needed to reflect an evolution in information.
Another aspect that relates to this sort of replacement potential (which can happen at any time) is the overall intention or use to which a label may be put. Even though some sort of basic information may seem set at any one time, it is rare that such a system of 'categories' is actually fixed (just ask those who are digitizing a collection to meet certain broader circumstances!). 
At the same time, labels can be used to contextualize items, to engage viewers, to help create a visitor experiences, etc. This sort of text, perhaps short, will change, and probably should. 
Labels are usually not 'fixed' for long periods of time, but reflect changing circumstances and intentions, both internal and cultural.

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