The reasons why I'd consider 100% mirrored boards to make little sense are two-fold:
1. You commonly learn using a computer with - guess what - a regular keyboard. As alternative keyboard layout users will certainly be able to attest, "unlearning" a layout is hard. Mirroring the whole alphanumeric section wouldn't make it any easier. Similarly, people would be used to cursor keys on the right even if they're lefties. Operating a keyboard is mostly "software" and very little "hardware". The finer details like lefty/righty only start to matter if you aim to maximize speed / comfort / efficiency / whatever.
2. MF-II and derived keyboard layouts aren't perfect for either right-handed or left-handed people to begin with, not in combination with a mouse at least.
I find it interesting that the ideal keyboard partly depends on what people are already used to. That's a nice little feedback loop right there. Worse yet, one with considerable time delay. Those into control theory will probably know that the result can be pretty messy.
Anyway, I think it's about time to rethink the keyboard layout, and do it properly. Dropping the poor numpad entirely is a workaround that goes in the right direction, but it's not a 100% solution.
One of the main problems certainly is point 2. The MF-II layout was conceived 25 years ago, when things relied much less upon them little pointing rodents. Nowadays we basically have 4 different groups with different demands:
1. Righties with right-handed mouse usage. Overworked right hand, large mouse - keyboard alternation distance. Classic target group for tenkeyless boards and separate numpads.
2. Lefties with left-handed mouse usage. Overworked left hand, moderate mouse - keyboard alternation distance. May have limited appreciation of cursor keys and numpad on right. For them, having the cursor keys and Ins/Del block (mirrored, I think) on the left plus a separate lefty (mirrored) numpad would be more useful.
3. Lefties with right-handed mouse usage. More balanced workload LH/RH, large mouse - keyboard alternation distance. Those would appreciate a mirrored (lefty) numpad to the left of the alphanumeric section, while the cursors could remain on the right.
4. Righties with left-handed mouse usage. More balanced workload LH/RH, moderate mouse - keyboard alternation distance. Those should get along with the regular layout fine (phew!), even though some might prefer the middle section with the cursors and stuff to the left of the alphanumeric section.
Let me conclude:
1. You have all 4 groups covered well when offering both "lefty" and "righty" tenkeyless boards plus corresponding separate numpads. (Having an alpha section plus detachable middle section with software-controlled mirroring function would essentially do the same as the two tenkeyless variants, but eliminate the need for keeping two different tenkeyless sections in stock.)
2. You have 3 groups covered well and one covered OK with a "righty" tenkeyless board plus both "lefty" and "righty" numpads.
3. "Lefty" numpads appear to be sorely missing. I (group #3) can operate a regular one OK but this never struck me as extraordinarily attractive.