I believe the letter is quite important; if your school offers a committee letter and you choose not to or are unable to receive that committee letter, schools will wonder why and you have to formally explain. I am of the udnerstanding that you need that committee letter, because not only are they able to summarize and pick out the highlights of your LORs, but they can add lots of good information about you and your performance at Penn.
I have nothing but great things to say about the Penn program. It is flexible, has an outstanding reputation, is in an area where plenty of clinical/volunteer opportunities are available, and will not send you to the poorhouse. There is less hand-holding than other post-bac programs but that encourages intiative.
Jon - as for the HPAB - it works different for post-bacs than for undergrads. Ugrads go through the process that docquaker described; post-bacs and CGS'ers don't have an interview. There is a questionaire, and you get 4-5 LORs, rank them in your preferred order, then submit this rank list and questionaire to the advisors where they will write your letter upon completion of 6 courses. Its an important letter, believe it or not.
Any other questions, PM me. Hope this helps.
youcandoit.