Does negativity have a place in marketing content? Sure it does! In fact, complaining about some problem or other is a time-honored tradition, especially in your pain statement. ("You scrubbed those dishes till your hands were raw and the plates are still filthy. Don't you wish there was a dishwashing liquid that really made things easier?") But like most copywriting techniques, there's a right way and a wrong way to go about it.
Consider the ever-popular political technique of tearing down the other guy through negative advertising. If you want to position yourself as better than the competition, go right ahead -- but is it really such a victory to be the best choice in an awful field? Consider whether it might make a stronger impact to focus why you're better than the other guys, not why the other guys are terrible. Constant slamming of Brand X might even come across as insecurity. The reader of your ad may ask, "Why doesn't this brand talk about its own positive qualities more? Doesn't it have any?"
Of course a pain statement can enhance the power of your marketing content by making your prospective customers feel that you truly understand their challenges, fears and frustrations. But even the most compassionate pain statement can work against you if you hammer away at it like Debbie Downer. Present the problem, yes, but then leap into the solution in time to salvage the audience members' mood and get them excited about the solution -- you.
I've mentioned previously that the occasional rant can add a little spice to your blog among the many other kinds of articles you post there. But tread carefully. Just as too much spice can ruin a meal, too much ranting can ruin your online presence. Negativity in large doses will simply drive readers away. I follow a certain well-known copywriter's blog less frequently than I once did for this very reason -- I got tired of reading complaints about this or that bad experience with a client, how the industry isn't what it used to be, etc. If a blog can be said to take on a persona, this one had become a cranky old fart. Don't you do the same, unless your business expressly caters to that crowd.
What about replies to complaints? Perhaps your business has a listing on some similar local-search products and services directory. If so, hopefully your listing collects rave reviews from satisfied customers -- but the occasional angry, name-calling criticism can go a long way toward wiping out your good reputation. The urge to post an equally vicious rebuttal may seem natural. Don't do it! Instead of descending to the level of the complaining post, to take some time to cool off and devise a rational, polite, professional-sounding response. If you can't manage to do that in a timely manner, hire a copywriter to do it for you. I've actually written several such rebuttals in the past for business owners who didn't trust themselves to respond in the right tone.
Positivity and negativity are two sides of the marketing content coin. Just remember that too much negative verbiage in a marketing piece is like too much negative space in a painting -- in both cases, you risk ending up with nothing.