You must already hold a MEP(LAND) class rating before commecning the ME IR, you can combine the multi with the CPL but not the IR. You are right if you complete a single IR then convert to the multi ir you need to take the full IRT again which is more than most people want. Also, the majority of the people would rather have the experience of the additional time on the twin, not to say having around 25 hours multi (using an FNPT I rather than an FNPT II) in your log book can be very attractive.