I'd like such a feature that changes the transparency (with a button or hotkey or whatever) of the view--part, body, feature, whatever--but not of the true material. This can be more handy than using HLV for the same purpose, since complicated models can make a mess of the display with all the lines. Anyway, it would be a much-faster toggle in the context of a part. Go to an assembly in which the part is used, and you have your original material displayed, not affected by the transparency toggle.
Again, this is just a means for simpler work. The current transparency setting applies the transparency in a "permanent" state to your material. Plus, making the change is a kludge of mouse clicking--which isn't conducive for quickly checking geometry through the part while constructing (for example) a sketch. Meintsi mentioned a perfect example of what sort of context this would be useful.
Also, if this is applied as a temporary toggle of the display of the part, possibility of confusion by the user is low--the toolbar will clearly indicate which "mode" the viewer sees, and clicking a different mode will immediately change it back where it was. If this was a true transparency button, I could see confusion, since we don't need to increase the avenues possible in doing so. But we don't currently have a temporary transparency "display" mode at the part level as we do on the assembly level. (I once threw a part into an assembly by itself just to get this capacity through the use of the assembly's part transparency toggle--worked faster for me, plus I could immediately either see through the part or not.)
Jeff Mowry
What did you dream? It's all right--we told you what to dream.
--Pink Floyd, Welcome to the Machine