Why should I use Google's CDN for jQuery?

This is because:

1. It increases the parallelism available.
   (Most browsers will only download 3 or 4 files at a time from any given site.)
   
   
2. It increases the chance that there will be a cache-hit.
   (As more sites follow this practice, more users already have the file ready.)
   
   
3. It ensures that the payload will be as small as possible.
   (Google can pre-compress the file in a wide array of formats (like GZIP or DEFLATE). This makes the time-to-download very small, because it is super compressed and it isn't compressed on the fly.)
   
   
4. It reduces the amount of bandwidth used by your server.
   (Google is basically offering free bandwidth.)
   
   
5. It ensures that the user will get a geographically close response.
   (Google has servers all over the world, further decreasing the latency.)
   
   
6. (Optional) They will automatically keep your scripts up to date.
   (If you like to "fly by the seat of your pants," you can always use the latest version of any script that they offer. These could fix security holes, but generally just break your stuff.)

There are several scenarios when you might not want to use jQuery from Google's CDN:

1. When you are building an intranet application where the web server is hosted on the same network as the clients. If you use Google's CDN jQuery, you will be making a call to the internet rather than a webserver on the local network. This increases bandwidth for your organization, and is slower.

2. When you want to run your application offline. (Quite linked to the first issue) If you need to work on a development environment (managed for example with Bower), you might need to be able to make your application work without any internet connection (ie: in a train :)

3. When you need to customize it. For example if you use Grunt to build the library in order to use only certain modules or setting the AMD name

4. When you are serving pages over SSL that require jQuery. You should serve the JavaScript over SSL as well as your page to avoid security problems and warnings.

Also, Microsoft hosts jQuery on their CDN. That is another choice comparable to using Google's hosted jQuery.

This study by TJ VanToll convinced me that it's better to concatenate jQuery with other scripts rather than load it from a CDN.

The reason is the latency involved in fetching jQuery on mobile devices:

"In 2012 the average RTT time on a mobile network in the United States was 344ms. And that 344ms applies to not only every HTTP request – which the average web page now makes 93 of – but also every DNS lookup and TCP connection... While average RTTs are improving, there are only small additional gains to be had, as current networks are within a small factor of the theoretical limit dictated by physics."

He also cites this post from Steve Souders that shows why you're actually unlikely to get the caching benefit from using a CDN:

"Because of the fragmentation in CDN providers, jQuery versions, and protocol usage (http vs. https), the chances of getting a CDN cache hit are shockingly low – and downloading from an external domain has the potential to perform not one, but three round trips (a DNS lookup, a TCP connection, and an HTTP GET)."