The team (i.e. Maureen) has quietly been making progress on integrating Domino Designer with Eclipse. You can start to see what benefits you will get in this picture of a LotusScript library. Note the outline on the right side of the screen! This should provide some tangible results for why to bother with this work. Don't get hung up on Eclipse - if you don't care, then fine, you don't have to. If you do care, and use Eclipse for other projects, then you can have a single environment from which to work.
The usual caveats apply - no ship dates have been declared, it may never happen, or it may look different in the end.
(Maureen has another picture on her blog. I didn't know whether she was going to post something or not...)
LotusScript library example - Domino Designer in Eclipse