Last time ITEM.DESCRIPTION is a nice editable value for item, which can be used in different ways.

Why? -> already long time (after implementing LLD) I'm thinking how user can distinct created items, when discovery rule can receive only a numeric {#SNMPINDEX} macro and its value. It's usual common case.
See for example on the wiki http://www.zabbix.com/wiki/_media/doc/speed_fan.png?cache=
Yes, there are graphs, which names can't be customized by design, but when an item name will be customizable then it can be distinguishable in a legend.
Items and triggers will be distinguishable in their lists.
Using this macro in the trigger names is useful on view list of events or triggers.

If item description is empty then ITEM.DESCRIPTION should be expanded to nothing.

I think this solution is replacement for the ZBXNEXT-577 and it can be performed after resolving ZBXNEXT-821.

But for templated (not LLD) items and triggers this feature can be useful too, with the same efficiency.

Heh, it's not easy explain my vision, if you have question I'll answer.