Is it ok to use the PCB as part of the structure of a product?
PCBs are used as structural materials in many applications. If you make sure you load it in the right way - no bending loads, only tensile - it can be a very strong and stiff material useful for many applications. Also, you can put a lot of very fine detail in PCBs, allowing you to delegate a lot of mechanical complexity to a very cheap process. This may improve manufacturability and lower cost of your other mechanical components.

If you are intending to use PCBs as structural materials, make sure that:


You keep in mind that rule number one of mechanical design is: orthogonalize the design. The easiest way to check this is to make sure that you can assemble the entire mechanical design without needing a million hands holding various different parts in place while you screw a certain part in place. Every step in the assembly should build upon the previous steps in a linear fashion and the end result of every assembly step should be a product that can be easily handled and manipulated.
Even though you are using a PCB as mechanical as well as electrical interconnect (and thus your design is not orthogonal), try to decouple the functions as much as possible anyway. Do not lead mechanical stresses through (densely) populated areas, as the stresses may deform the PCB and cause microcracks. Use slots in the PCB intelligently to lead mechanical stresses around the populated area, through unpopulated 'less important' PCB material
Use sleeved fasteners or very fine pitched threaded fasteners in your PCB, DO NOT use self-tappers. The PCB material as a whole is very strong, but the insides of unplated holes are very easily damaged, compromising the stability of the connection. 
Apply solder to the annular ring of mounting holes and use serrated rings to self-lock the fastener in place.
very important: use vias in the mounting hole pads to 'nail down' the copper onto the board. Otherwise the annular ring will easily come loose under mechanical stress.
Use appropriate board thicknesses and do some back of the envelope stress analysis. Under typical conditions you want less than roughly \$\epsilon = 0.001\$ strain on your circuit board. This is combined thermal and mechanical. Using the mechanical properties of your chosen board material, calculate the amount of strain you expect in your application. Thicker boards means you can take up proportionally more force for the same amount of strain. 
In applications where excessive strain is unavoidable, route your traces with round corners instead of sharp edges, use the smallest available component packaging and orient the packages in the orientation that can take up the most strain. Leaded parts can cope with more strain than leadless parts.