This thread stems from a bug reported today (see http://bugs.archlinux.org/index.php?do=details&id=2734)
The crux of the story is that I ran into difficulties upgrading Eclipse via pacman. This is because I had already installed new plugins, and updated existing ones via Eclipse's own update system.
Now, if I were to remove the old package, and then re-intstall the current version of Eclipse, I would lose all the new plugins that I'd taken the time to install. For example, the Visual Editor and its dependencies. I'd rather not have to install these each time there is an upgrade to Eclipse in the Arch repos.
So, there needs to some discussion about what approach to take for these special cases. The obvious approaches are:
1) Put Eclipse on the IgnorePkg once installed and update through its own means.
2) Package more of the common Eclipse plugins and add to repos, and keep them all up-to-date via pacman.
The first is easier, but the second fits better with pacman.