How limiting are web frameworks
If you know you're not going to use an ORM, or create a user interface, then you've just eliminated about 90% of what you'd use a web application framework for in the first place.  If you look at the feature set of Django, for instance:  what parts of that would you use to implement a web service that you couldn't get from using something much simpler, like Werkzeug or CherryPy?
The principal differences between building a web service and building any old black box that takes input and produces output are the various technical limitations imposed by the API being HTTP-based, the problem of statelessness, and the problem of idempotence.  A web application framework is going to give you a little help with those issues, but not much.
This is a general question about how limiting are web development frameworks such as Django and ruby-on-rails.
I am planning on building a RESTful web service which will have a purely JSON/XML interface, no GUI. The service will rely on a database however for a few of the more important operations there is no clear way of persisting a "model" object directly into a database table. In addition I require full control over when and how the data is being written to the database. I will need to maintain multiple database connections in order to use some connections only for reads and others only for writes.
I've looked at the "full" MVC frameworks such as Django and more basic ones such web.py and pylons. The impression I currently have is that if I go with the full framework initially things will go faster but eventually I will get stuck because I will be limited by the framework in what I can do. If I go with a more basic framework it will take much longer to get everything running but I will be free to do what I need.
This is what it seems like but I suspect that it might be an incorrect impression given how many sites are written in Django and Rails. Could you please provide your opinion. Am I totally wrong and there is a way to easily do anything with a framework like Django or Rails or given my requirements I should go with something like web.py?
Thank you!