True, it is a "common" mistake. It is also amateurish.
I run a few sites on MT. We pay the ~$20/mo hosting fees with the expectation of well-configured apache servers, fair customer support, and the ability to handle an occasional Slashdot, Reddit, or Digg effect. While we keep our CMS' up-to-date, we are happy to pay for the overhead of having someone else deal with OS, network, and server issues.
Unfortunately, their "amateurish" mistake pretty severely attenuates our confidence in their competence -- for existing and future features alike. Worse, it equates to real lost time for us to recover, additional lost time as we implement new security measures (e.g. Tripwire IDS), and frustration with the fact that we found out about our sites being hacked back on Nov. 12th and yet they are just now notifying us of the larger issue!