I feel like we're stretching Excel further than it really should go because it hides a lot of complexity and doesn't allow for easy testing. Couple this with the fact there doesn't appear to be a large open source movement around Excel models so fewer people can verify its correctness.

Here's another a Reddit thread that inspired me to ask this question: http://www.reddit.com/r/finance/comments/35esdl/inside_the_johns_hopkins_finance_class_thats/