But I am wondering what are good reasons to buy this instead of the Custom 74 or Heritage 91, since they are not much more expensive in Japan and they have more appeal as a next-level pen.
I use an "M"-nibbed Custom as my kitchen grocery list pen, filled with Sailor sei-boku pigmented ink. It is a better solution than the 74 or 91 in that situation because of its snap cap (nobody in my family wants to spend time unscrewing a cap when they want to quickly write down something in the middle of cooking, say) and because its more simple section is more easily and quickly cleaned (an advantage when using pigmented ink).
We use the Custom instead of a Prera "M" in the kitchen because the Prera unposted is too short for my husband's hand (and again, taking the time to post the pen isn't a happenin' thing in our kitchen).
We use the Custom instead of a Metro as a kitchen pen because the Custom's cap can be easily disassembled for cleaning (again, because of the sei-boku, I like to stay on top of that).
We use the Custom instead of a 78G as a kitchen pen because I don't like plated nibs, so I don't own one.
If I could get the Custom as a demonstrator, I'd be thrilled. I'd also like the option of a FM nib.