I'm not quite sure when I should use SingletonScope() vs TransientScope() vs RequestScope() when I do my binding in my global.cs file.
I have for example my call to MongoSession (using NoRM and the mvcStarter project http://mvcstarter.codeplex.com/) which is set to SingletonScope but I created a repository that use this MongoSession object to make calls to Mongo easier, e.g., I have a NewsRepository which uses MongoSession to fetch my News items from the data. As an example I have a call that fetches News items that has DisplayOnHome set to true and get the latest by CreationDate. Should such a repository be SingletonScope or would RequestScope would be more appropriate?
When should I use each of it and why?
In general in a web app, you want state to be request scope as much as possible.
Only in the case of very low level optimisations are you ever likely to run into a case where its appropriate to create singleton objects (and the chances even then are that you'll pull such caching / sharing logic out into another class which gets pulled in as a dependency on your other [request scope] objects and make that singleton scope). Remember that a singleton in the context of a web app means multiple threads using the same objects. This is rarely good news.
On the same basis, transient scope is the most straightforward default (and that's why Ninject 2 makes it so) - request scope should only come into the equation when something needs to be shared for performance reasons, etc. (or because that's simply the context of the sharing [as mentioned in the other answer]).