I've noticed a lot of Hubs do not link to references. Is this against the rules of Hub Pages? I'm used to including references at the end of my articles, but I can leave them out if it's not allowed here.
There's no rule here about them. Free free to use them if you think they are appropriate.
IMHO, unless you're writing a scientific/medical article, I don't think they're exactly necessary. If you're writing a "How To" or other informational piece, resources to give the reader more information are always helpful, but I would hyperlink appropriate words in the text rather than give citations at the end. Just my opinion though...
You can add any references you choose to add that are relevant to your topic. You are not required to do so, but it does give your reader something else to reference and your reader might find more value in your work more so than without it.
However, if you cover your topic thoroughly then you have no need to reference anything else.