Libraries are not only useful, but necessary in VHDL. The IEEE libraries, for example, are used in virtually every design. Other libraries are used by synthesis tools to allow device-specific modules to be synthesized. User libraries can be useful too. They can be a collection of tested, frequently used modules that can be incorported in designs without your having to remember exactly how they are implemented. However, the method for making user libraries available (by reference in a library and use statement) is design-tool dependent. I am familiar with how to do this only for Xylinx ISE and Aldec Active-HDL. If you need direction for either of these tools, I can help you; otherwise someone else may be able to give you guidance.