In general it's better to have a single combined Javascript file. But if you have a lot of JS that only applies to one page or section you could keep that in a separate file so the other sections don't get slowed down.
I think Q2A keeps the jQuery library separate to allow you to use your own version, if you for example already use it on the rest of your site. Or you can swap it out for Google's CDN-hosted version.
The other stuff is quite small so it ought to all be in one file.
