I wouldn't agree with this 100%. I principally use jQuery unless I am working on something that has to very lightweight. However, there are lots of libraries and different reasons people and companies use them. That's why it's important to know the language in detail so that you can switch between libraries as needed.
Right, there is also Dojo hanging out there for large browser Apps. There is a point in which you can outgrow jQuery due to the size and complexity of an application. Dojo kind of fills that upper end so while it is a smaller segment of the market, there are still other toolkits that hold their share of the market for what they are tailored for. In saying that, there are some newer, complementary projects like backbone that are trying to make large web app development in jQuery less painful, but as of right now Dojo owns that space and it remains to be seen if that will change.
