While I don't support a blanket statements along the line of "every app should be using MongoDB," it is equally invalid to say that "anything that deals with money" has no use for MongoDB. If you look at http://www.mongodb.org/display/DOCS/Production+Deployments you will see a few financial and ecommerce sites. I can tell you that there are even more financial firms not on that list in various stages of production. Even if your entire app can't be written using MongoDB, it is still worth investigating if the time savings of using it for a portion outweigh the costs of using an additional data store. But that is more of a business question than a technical one.
Could you provide some examples? Scanning the list I see a couple that have a very periphery relation to financials, but the actual applications have very little financial applicability (and the implementations are trivial).
Though the person you responded to didn't actually say that `"anything that deals with money" has no use for MongoDB"', so you're setting up a strawman regardless.
In that case, I apologize. I have seen people using the same argument to claim that MongoDB is unsuitable for use in ecommerce and assumed you were doing the same.
