True, but instead of adding "/" to the source you obtain the same effect by deleting the last component of the destination (which is normally a non-action, you just do not auto-complete far enough).
If you meant that you might want to have "foo" and "bar" directories with the same content and timestamps, but with different names, you can rename the target directory before and after the rsync.
This is negligible overhead, because in many decades of using computers for all kinds of applications, I have not encountered a single case when I would have needed to have 2 directories with identical content and even with identical directory timestamps, but with different directory names.
Making a backup directory in the same directory with the original directory would not be a good example, that would be just stupid. Any backup directory should be placed as far as possible from the original directory, when not on a different filesystem, in which case they can have the same name.
The way how rsync handles names without trailing slashes is completely right.
The way how rsync handles trailing slashes to encode alternative behaviors might have been OK if this feature would not have been in conflict with the auto-complete feature of the shells.
For all the rsync behaviors that can be obtained by adding some "/", there are alternatives almost as simple as that.
A slight simplification in the encoding of the seldom used alternative behaviors is paid by a large complication in the typing of the frequently used behaviors, where you must always insert a backspace + space sequence after each directory name, to delete the dangerous trailing slash.