1. Testing enables you to certify your design. To certify means that you are validating your design in objective terms of quality, safety, and functionality. You might have seen the different groups out there that deal with design quality like ISO, DIN, IEC, NEC, ANSI, CAS, ASME, and other "alphabet soup" agencies.
2. Testing allows you to proactively screen your design against trolls, customer returns, and bad reviews. The more you test, the less you are leaving to chance. The more you mitigate your design risks the less of chance you'll encounter issues.
3. Testing is not the same as assembly. Assembly is the physical assembly of the prototype or final product for future testing. Please don't get sucked into the school of thought that assembly is a substitute for real world testing. Testing is different and it should be viewed as such.
4. Testing doesn't always have to take the form of labs, wind tunnels, and other traditional testing areas. Focus groups can provide reviews, complaint-worthy issues about your design, and give you some constructive feedback. In the end it's better to have a focus group negatively review your design than have a real customer do it.
5. For those that can't afford a focus group or certifying agency consider asking trusted friends, family members, or even coworkers. Just ask them, "Can you please review my product, use it, and tell me what you think? Don't worry about hurting my feelings or offending me: I just want to know exactly what you think." And try not to be too critical of the feedback, but rather objectively evaluate if their feedback will help you to be more profitable.
6. I have found that it's best to test designs in "worst case scenarios." As Murphy's Law says, "If it can go wrong, it will go wrong" so don't leave anything reasonable to chance and make your future more secure. Don't leave anything to doubt so test, test, test, and test until you have no doubts about your design! Finally, for every defective feature you find in your design is one less product returned, one less bad review, or less day troubled with a customer complaint.
To summarize: when in doubt, test!