I've just found git-subtree tool that some time ago became a part of main git repo https://github.com/apenwarr/git-subtree/
However I don't fully understand what functionality does this tool provide over the already existing "git read-tree" + "git merge -s subtree". Is the only purpouse of git-subtree - making the resultant commit history look better or does it have more functionality that I've overlooked?
The commands you describe reads a subtree into a repository. The git-subtree command has many more options, as described by the documentation. Among others, you can (annotated for simplicity):
add::
Create the <prefix> subtree by importing its contents
from the given <refspec> or <repository> and remote <refspec>.
merge::
Merge recent changes up to <commit> into the <prefix>
subtree.
pull::
Exactly like 'merge', but parallels 'git pull' in that
it fetches the given commit from the specified remote
repository.
push::
Does a 'split' (see above) using the <prefix> supplied
and then does a 'git push' to push the result to the
repository and refspec. This can be used to push your
subtree to different branches of the remote repository.
split::
Extract a new, synthetic project history from the
history of the <prefix> subtree. The new history
includes only the commits (including merges) that
affected <prefix>, and each of those commits now has the
contents of <prefix> at the root of the project instead
of in a subdirectory. Thus, the newly created history
is suitable for export as a separate git repository.
There are also a variety of flags that aid and manipulate the above. I believe all these options were available before via chains of plumbing commands. git-subtree.sh just wraps them and makes them considerably easier to execute.