As Geany is an IDE the basic use-case for autocomplete is for programming language identifiers where there frequency is meaningless and alphabetic order is appropriate.
The autocomplete all words is a simple extension of that functionality that allows completion for filetypes that do not have parsers. I presume you are referring to this mode, then "useless" would be to not allow it at all, providing an alternative sorting order is an enhancement which may be useful.
If somebody wants to provide an option for counting the occurrence frequency and sorting on that, then if can be considered.