Does complaining have a place in marketing? 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 as with most copywriting techniques, there's a right way and wrong way to go about it.
Tearing down the other guy has its downside. If you want to position yourself as better than competition, go right ahead -- but I wouldn't make a whole campaign about it. Your audience might wonder why you're feeling that insecure about Brand X. The reader of your content may ask,"Why doesn't this brand talk about its own positive qualities more?" Even your 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 reader's mood and get them excited about the solution -- you.
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 the "You kids get off my lawn" crowd.
What about replies to complaints? Perhaps your business has a listing on Citysearch or some similar local-search directory for products and services. 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.
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.