You may have unacceptable values in a combo. For example, a product out of stock.
If you do not put unselectable items in the combo, your customers will say you: “Why can’t I see raspberries in the list? I’m sure I added them to the system“.
And if you put everything in the combo, they will say you “Oh, that’s too much, why do I see all this useless stuff?“.
This plugin for Ext.form.ComboBox grays out items you mark as unselectable:
Full presentation -- Demo & download
I hope you will find this useful!