There's another significant point. The IMC written is tough and requires proper study. (It's a 2 hour written) As such it's a great insight into the level required for the IR or either set of commercial exams. We once had an IMC student fail the exam having very recently passed the ATPLs! Of course a written IMC pass wasn't actually required as the ATPLs were sufficient for rating issue, but it was an eye opener for the candidate.