'Store' in this context sounds very much like a [Repository][1] or [Service][2].  In that case, this is an extremely common pattern.  **The flaws/problems will vary with your implementation and the problem domain.**

On a general level, it sounds like the book is using 'Store' to represent a level of business logic + a level of data retrieval logic that handles a set of data that may or may not be part of your application.

For instance, wrapping the twitter api in a 'Store' is a nice way to compartmentalize that logic.

**Upon further thought**  
Using [this definition of MVC][3] (which I think is pretty spot on), the 'Store' is really a subset of the model.  The delineation between whether it's an extension to MVC or whether it's a data retrieval pattern isn't terribly useful.  They end up looking like the same code.

Bottom line, I think you'll be fine following the advice they suggest (seems sound overall).


  [1]: http://martinfowler.com/eaaCatalog/repository.html
  [2]: http://www.corej2eepatterns.com/Patterns2ndEd/ApplicationService.htm
  [3]: http://programmers.stackexchange.com/questions/127624/what-is-mvc-really/127632#127632