A possible reaction to a critical review of anything is "Don't be so negative" or "Why can't you be more positive." I understand the function of the comment. It hopes to help a person to focus on the "half full" aspects of a situation, and for many practical purposes, it is very useful. For example, when caught in a rainstorm without an umbrella, why not just enjoy the rain. A positive attitude would be "heck, I'll just dance," while a negative attitude would be "this always happens to me and now the day is ruined." The positive mindset will involve accepting situations outside your control, which involve many in our daily routine. What is not often discussed is that a positive mindset, in specific situations, is more wishful thinking than historically or empirically supported evidence.
So, then, how does a negative mindset become beneficial? The guiding overarching recommendation is to use it to improve a situation and/or protect yourself from manipulation. Think of it as looking at the glass with water and saying "Hmmm...it's half empty, so how do I get it to be completely full." In this example, negative thinking is to never be satisfied with a particular success, but to help focus on successive improvements.
A negative mindset can be successfully applied to many everyday experiences. For instance, to be skeptical with advertisers. Television, radio, or online commercials are constructed to get your attention for the purpose of selling their product, which in all likelihood will have no observable improvement in your life. Hoping the "new" product provides an improvement is positive thinking, but more likely than not, it is only wishful thinking. To counter, try self-talk. Self-talk when subjected to advertisers (or any unwelcome salesperson) is something I do, especially with pharmaceutical commercials. When I so happen to be subjected to one, and it gets to the place in all commercials that say "Ask your doctor about Blahblah," I will say, in my head or out loud, "No way Jose" or "Don't tell me what to do." It sounds negative, but such self-talk will counter the efforts made to get you to spend money or cause you unneeded anxiety.
If there is no evidence for making a change, then don't change, which may sound negative, but your decision is based on evidence and not on the imagined possibility of a better product or behavior. For example, this is often observed in relationships. There are often promises, and a positive mindset will give the benefit of the doubt, but a negative mindset says "I don't have confidence you will do it so I need to see a plan and short term expectations. I don't think you can do it by sheer willpower." Another application of a negative mindset is on the function of psychotropic medications, such as "do I need this while I'm grieving?" or "what is the cost/benefit on my brain in the long-run?" Be skeptical with "preventative" use of medications. You are not being negative or a "bad patient" but protecting yourself and your interests.
A positive attitude is a wonderful way to address the uncontrollable aspects of our lives and keep our mood stable in difficult times. However, the negative attitude will serve to protect ourselves and interest when the evidence suggests a better alternative is needed, or to keep us protected against manipulation.