To add a bit to the fray here, one thing I've noticed is that not all elements ("nodes") defined in a given XML entity are used by the UIengine. Further, not all parameters of various elements of the UI_<name>_<server>.ini file are applicable to all windows.
Example:
I was playing with eliminating the casting window and moving the casting bar.
My first attempt was to look at the .ini file, where I'd noticed a "Show=0" entry on certain windows that I had disabled from within the game. I added that entry to the [CastingWindow] section. Loaded affected character and guess what? No dice. I cast a spell and the casting window popped up with a casting bar in it.
I then modified the XML - I set the <StyleTransparant> value to true for the casting gauge and the CastingWindow ScreenItem. Tried that, again, no dice. I got a window and a casting meter.
I then went in and set the <ShowBorder> value to false. That worked somewhat, this time I had a window with no border, but a casting gauge still appeared in the screen.
As noted above, setting the size to 0,0 effectively eliminated the window.
Based on this and some other things, here's what I think is going on. The UIengine is using the XML to define values for various screen elements. To the extent that the engine's behavior for the default configuration doesn't change those settings, our modifications will hold. The UIEngine adjusts colors and <Show> properties for the elements as the game runs. Hence my default values for not showing the CastingWindow not working. The default for that window is "don't show" - except when your casting a spell. The UIengine overwrote my values to perform as it was intended to do - to show the window when the casting was occuring.
Further, as is noted, the color of some <EQType> labels change based on game events - noteably the Player HP and MaxHP, AC, stats and saves - which turn red if you are debuffed, white normally and green if you are buffed. These cannot be controlled by the XML since the values loaded are changed dynamically by the engine while the UI is running.
I think one factor that may control the necessity of a given <Pieces></Pieces> callout is whether that element is dynamically toggled by the game (like the appearance of the PetHP gauge) or if the element is used for UserEntry (like clicking on the PlayerHP gauge).