We're adding 'enterprisey' features because our customers need them (interfacing to accounting and purchase order systems). It's not just for the sake of it.
We have a very vertical market and a small number of clients: some of our clients have customisations they want to allow our system to integrate with other systems, or to handle business logic that is unique to them. Using MEF will allow us to have customer-specific portions of code that only get installed for the specific customer who commissions them -- it's like a 'semi-bespoke' system. We won't be opening up for third parties to add extensions, we will be coding it all.