We always have a fair amount of come and go. People often stop being imms for many of the same reasons people stop being players -- their real life situation changes, they need a break, whatever. Most of these leave on good terms, and sometimes they come back in some capacity.

For example, Aarn was on his fourth distinct "tour of duty" as an imm, Ordasen his second, Dalteric also his second. I'm talking separate periods of being an imm with year or more breaks away between, not just different incarnations / imm characters.