See the attachment. With ansible 1.7.2 this works as expected:
% ansible-playbook --version
ansible-playbook 1.7.2
% ansible-playbook -i hosts/localhost a.yml --tags b
PLAY [localhost]
(attachments)
x.tar.gz (308 Bytes)
On this subject, we've found a bunch of regressions where ansible's
playbook semantics (especially when it comes to the behaviour of variables
in included roles) don't appear to be nailed down.
Is there a regression framework for test cases? I'm happy to submit a set
of playbooks that illustrate the variability we're seeing. At the moment
we're kind of in the position of having to choose which set of bugs is the
least egregious 
Cheers,
jan
There is an open ticket already on this issue
https://github.com/ansible/ansible/issues/9862.