I agree. The lack of 4 cable capability of the M9 almost kept me from getting it. I wouldn't have minded dropping a few more bucks, but the M13 just takes up way too much real estate for what I need.
The M9 is designed to be used in front of the amp, but the first year I had it, I used it in the loop. Obviously this kept me from taking advantage of a number of it's effects, but the delays and mod effects are much nicer in the loop. Unfortunately, there was too much trade-off, so I now use it in-line as it's intended. Ya can't have it both ways with M9.
Phil's correct - you need 4 cable handling to accomplish that - something M9 does not offer.