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As far as I can tell the Meta and Control digit-argument bindings (C-1, C-2, C-3, M-1, M-2, M-3) are duplicates.
There are so few single-modifier, single-key bindings available, it seems like a perfect opportunity for personal bindings.
Is there any special reason one shouldn't use, for example, all the Meta-digit bindings for their own needs?
Yes, they are duplicates. If you want to use one of them for your own purposes, I would recommend to keep the meta ones and rebind the control ones. One reason to keep the meta ones is that if you run Emacs in a terminal, most terminals don't transmit digit differently from plain Ctrl+digit, which makes Ctrl+digit bindings inaccessible. On the other hand text terminals transmit Meta+digit as Esc digit. Another reason to keep the meta bindings is that they are the same bindings that let you type Esc digit…, since the Meta (M-) modifier is equivalent to the
Esc (ESC) prefix.
If you really want, you can even rebind both, since the same functionality is also available by typing Ctrl+U and then digits without any modifier. So for example all of the following are equivalent and move the point 12 words to the left:
C-1 C-2 C-left
M-1 M-2 M-b
C-u 1 2 C-left
ESC 1 2 C-left
M-1 C-2 M-b
…
They are duplicates, indeed. The reason they exist is to make it more ergonomic to type digit arguments regardless if the next command uses Meta or Control. For example, typing C-5 M-f is very convoluted; M-5 M-f is much easier.
You are free to bind them to your own commands if you like.
