Wouldn't normally still be here but I needed to make an edit in the last post.

The reason for using /targetlasttarget is not really to remember the previous target but for target obfuscation in a PvP enviroment. When entering an arena each toon can be targetted on a different member of the opposition to make it difficult for them to tell who is about to be hit or even who your current leader is. It's also extremely confusing if you have an agro warning addons when every member of your team is being re-targetted after every single spell cast.

Buggy or not, an empty or re-aquired dummy target is extremely useful in PvP. I've never noticed a bug, although I use the /target seldomly as a secondary focus in a PvE group.