Having a detached numpad makes sense to me. It allows you to get the mouse closer to the keyboard.
Back in the day, I had a HHKB lite and bought it for that very reason. I never liked having to reach past the numpad to get to the mouse. The ergonomics seemed out of whack to me.
I ended up putting the mouse on the left hand side of the keyboard to negate the issue. Of course, I just severely limited the type of mice I can use.
But I do have a fair collection of ambidextrous mice. I suppose though that since I mostly use numpad-less keyboards I could put the mouse back on the right hand side. But it turns out that having the mouse on the left side has its benefits. It allows me to use my right hand to manipulate the arrow keys and page up/down.
Not so much on the HHKB, but it works great on the filco tenless and model m mini.