Sorry to disagree, but folks asking for completion seem to be assuming that users of Arduino IDE also use stuff with word completion.
That is not my experience, and the ones who do, most don't expect it in other programs. As long as it is off by default, I'm okay.
I believe it is still true that the majority of the worlds population don't use computers. I'd suspect that a large fraction of those who do use computers, don't use Word, or a programming environment. Arduino may be the first programming experience for a most of the folks. I know that is true for many of the groups I work with.
I'd still encourage completion-aficionados package the Arduino toolchain as a plugin for Eclipse so that you get all of the other features you like.
Please write down the stories, or use cases, and actors which benefit by completion. Then identify the actors or use cases which are not addressed by Eclipse-for-Arduino.
Just a thought.
GB