Of course this is a design decision and we can't ask the original maintainer of the package, since he is not active any more. As far as I can tell, the separation between the normal cite command and the multicite-command (with
s) has been there from the start.
The behaviour of the
\cite command in
biblatex largely coincides with the behaviour of that command with other packages. The multicite versions, however, offer additional features that not many (any?) other packages provide.
The syntax of the multicite commands is fully backwards compatible with that of the standard cite commands. So
\let\cite\cites will not introduce more problems than you would have if you used
\cites exclusively and never allowed yourself to use
\cite. (Remember to
\let all commands to their multicite counterparts, you don't want inconsistencies.)
I would strongly advise against making multicite the norm, though. The behaviour of cite and multicite commands in biblatex is so well-known and established that it should not be changed. Finally, the multicite commands are much more sophisticated and complicated due to their argument structure that it is better to only use them when necessary and not as default.
The set-up needed for the multi cite command is obviously a bit more intricate than the standard commands. That is why we define the standard commands first and then the multicite commands on top with
I assume that one of the reasons why the separation was kept is to allow to define a simple cite command first and then to top it off with a complicated multicite structure. This allows a layered approach.
The most compelling reason for maintaining the separation between the two, however, is that the multicite command is much more intricate and fragile. The nature of the multicite command dictates that it does not know the number of arguments beforehand. So it will have to scan ahead for things that look like arguments. Even though this is quite robust, there are situations where a more stable command that knows its numbers of arguments is preferable.
works, because the
\cite already has read all its argument and so knows that
["this"] can't be its argument. But in
\cites does not know that
["this"] is not one of its arguments. You need to stick in a
\ to tell it to stop scanning: