According to Paul Tuohy's style guide, and other sources I'm sure I've read before, the default activation group QILE should be avoided when creating ILE RPG programs. However, Googling this subject I can't seem to find out the reason for this.
According to this pdf presentation QILE is often used by the system, could this why it should be avoided?
Here is a reference to QILE being a poor choice of activation group but no reason given.
I think it was me who initiated the idea that choosing QILE was not the best idea. I stand by that, especially if you are a third-party application as Mark mentioned, or even if you do or might use third-party applications that use QILE.
If I didn't have any particular reason to choose one activation group over another, I'd have my own non-specific activation group rather than using QILE.