joining files with empty field

An easy way to join two text files by common column is to use the "join" command in linux.  Here are some examples for that: http://how-to.linuxcareer.com/learning-linux-commands-join

$ cat A.txt B.txt
John    A       1
Linda   B       2
Rares   C       3
1       A
2       B
3       C
$ join -1 2 -2 2 A.txt B.txt
A John 1 1
B Linda 2 2
C Rares 3 3

What the trick is, when there is missed/empty field in one of the file, the "-e" option works only when "-o" is set. Here is full story for that: http://stackoverflow.com/questions/984986/padding-empty-field-in-unix-join-operation

"Important: FILE1 and FILE2 must be sorted on the join fields." (from this online manpage).
This problem #1. Problem #2 is worse: option -e is badly documented -- only works in conjunction with-o, so for example:
$ join -a 1 -a 2 -e'-' -o '0,1.2,2.2' sfile1.txt sfile2.txt 

bar 2 -   

boo - z 

foo 1 x 

qux 3 y 

where the s prefix name indicated files that I've sorted beforehand.