why would I develop the application in c++?
For the love of it..
First, when you're developing something using .NET or MFC or J2XX or some such framework, the design of their application is [to
some extent] limited. If the designer is not too experienced I would say it's limited to a
large extent. In C++ on the other hand you have a free hand at that.
Secondly from my own experience I'll say this: Framework (MFC, J2XX...) based applications are bound to be slower than the ones written without them (C/C++, Core Java) as the a developer of framework has to consider a larger group of clients than the developer of a single application. Thus the developer of a single application will surely design something that's more fitting to his/her requirements.
Third reason is there are no frameworks for business logic part of applications, say if you're developing an application that connects a stock broker to a stock exchange and provides for all trading facilities, you'll have frameworks (e.g. .NET) that can make life easier to create a GUI and access to DB, but it can't provide you any help in implementing the rules/regulations of stock trading, so this has to be done in C++/Java/...
A word on STL, it's meant only for aiding in implementing your business logic. Take teh previous example, once you've read teh data from DB you need to make some decisions based on that by correlating them and applying some rules, to hold this data in memory and manipulate it you'll use STL.
Finally IMHO .NET is meant for interoperability of different languages/technologies. And that's what it should be used for.