Detecting a texture change is sometimes crucial for a scripted object, for example when a script needs to re-assert it's texture for a status display, or just to 'fix' changes. If a script in another prim in the linkset, or a another script in the same prim triggered a texture change, it can be downright useful for a secondary script to be able to detect that change, if only to 'act in unison' with that texture change.
I ran into this issue while working on a modding project, where I don't have access to modify the original script.
I have a scripted object that operates by changing the texture of a "TV screen", and running animated textures on the face of it. It also sets a PRIM_POINT_LIGHT on that prim. I'd like to disable that light, but there is no way to detect when a light is being enabled. We are supposed to have a way to detect changed textures, but this function seems to be broken for detecting scripted texture changes dating back to at least the Jira, and SVC-914. http://jira.secondlife.com/browse/SVC-914
The unfortunate alternative is that I must now run a timer, constantly checking for the presence of a light on the child prim, in order to disable it when present. Creating additional overhead that could have been avoided if there was a way to simply trigger on the event of the texture being changed by the other script, or the light turning on/off.