If I understand pull-lp-source's man page correctly, pull-lp-source is not at all a substitute for apt-get build-dep. The latter will install *and update* the relevant source code, so that your dependencies are as secure as the rest of the repository. The former provides a one-off download of the source code. You will not automatically be provided with fixes for that code.
@Julian Andres Klode:
If I understand pull-lp-source's man page correctly, pull-lp-source is not at all a substitute for apt-get build-dep. The latter will install *and update* the relevant source code, so that your dependencies are as secure as the rest of the repository. The former provides a one-off download of the source code. You will not automatically be provided with fixes for that code.