A few of my colleagues and I were discussing ways to maintain older versions of PHP (mostly 5.2.x) on newer Ubuntu releases. Being that the only versions in the repositories on 10.04+ are 5.3.x, the most common resolution is apt-pinning. My coworkers, mostly citing possible dependency issues in the future, seemed to agree that this is not smart, but I'm not too clear on it. I would imagine that there could be cases where apt-pinning other packages is fine, but with something like PHP it may not be the best idea.
Why would apt-pinning an older version of PHP be frowned upon?