I wholly disagree. It may not be your preferred way (say, .1,.2 but it is totally valid)
You could do
--backup-dir=back/`date +%s`
In fact, while not everything, you can come really close with:
--suffix=`date +%s`
(and maybe add --suffix-keep-extension). This is like --backup-dir but instead of a move to a new dir, it is just a rename.
And then you can apply the filters/rotation with calls to delete using --include (with [[:alnum:]] or other advanced filters) and --max-age.
I am very much not saying this is identical to what you ask. Just that you can get close right now