This is correct. However any bzr operation like a simple C-x v = fails
because bzr is locked by the commit (it doesn't seem to do read-only locks).
That is the main disadvantage that makes me go back to the task branch.
I agree that this is a disadvantage, but is it really that
significant? How much do you use bzr operations while developing?
Most of development is not about bzr, it's about editing, compiling,
and testing. I only need bzr operations when I'm almost done with
development, like to see what files are changed or compare them with
some other version. No matter how slow is "bzr ci" upstream, the
development cycle is much slower, at least for me. It can only be
faster if you fix very simple bugs, like typos.
YMMV, but this disadvantage doesn't sound a reason good enough to make
a cardinal decision to do all the work from a task branch.