I appreciate the question. Reasons are
1. I did not now about StatET and dont use eclipse.
2. I wanted something similar enough to RStudio to be able to share it with colleagues who use RStudio so they could quickly switch over.
3. I needed something where I could test and check set-up and config before integrating the code into a java project. Ride does just that e.g. I can use the Aetherpackage loader for convenience when developing someting, then switch to Classpath package loader and fix the versions I want in the pom that starts Ride so I know exactly that the code I run will work given the specified dependencies. This makes it very easy for java developers without much experience of Renjin to integrate the code into their project. Maybe this is an unusual requirement but at work we have a bunch of very good data analysts and data scientists who are good at doing stuff in R and SAS but know nothing about Java (and are not interested in learning). On the other have we have many Java developers who knows close to nothing about R. Thanks to Renjin these two groups can now collaborate on projects and Ride helps to facilitate that.
4. This started as something much smaller and then grew because it was fun to do it and I found more and more use of it as I went along.
The Renjin REPL is also a nice tool but it is not an IDE. OTOH Alex already did a plugin for IntelliJ which is pretty good and useful I think so there are options available. I think the more options that is available to help the uptake of Renjin use, the better.
That being said, StatET looks like a nice project and I think it would be good if someone added support for using Renjin in addition to executing in GNU R.