Selecting digital componentry in bulk
From my experience, distributors are prefered for small to medium production quantities. The main reasons for this would be:


No lead time. Distributors stock components and can deliver them in a day. When buying directly form the manufacturer, lead times of 14-weeks are not unusual and can get much longer for more obscure parts.
Safety. If you want to buy cheap components from China, you have to worry about counterfeiting. This involves having extensive testing procedure, involving x-ray scans and thorough functional tests.
Minimum order quantity (MOQ). Manufacturers normaly sell components in quantities of 2500-4000.


You would go directly to the manufacturer if:


You need a part that's under an NDA and is not sold via normal distribution channels.
You need a special service as a part of your purchase (e.g. factory programming of chips, custom marking etc).
The lead time is short and the price is better.


Also worth mentioning is the fact that Big Three (Digikey, Mouser, Farnell/E14) will not alsways be the cheapest option. There are a lot of smaller distributors that offer the same services (and more) at better prices.

EDIT: Adding some info on counterfeiting.

Counterfeit components can be placed in the following categories:


Completely fake and non-functional parts. You basically get an IC package and nothing inside. You can often use x-ray to check this.
Night shift. Basically the factory runs an extra shift and makes components for the grey market. They may skip testing to save time and costs.
Semi-"Functional" parts that pretend to be original. See this FTDI example.
Recovered parts. These have been desoldered from scrap boards, cleaned up and repackaged.