Let's break up the complaint into it's two components. First, the complaint states that atheism and skepticism are completely negative. This is not true. Skepticism goes hand in hand with science, which is hardly negative. Atheism often accompanies a ton of positive views (usually including science), though the particular positive views may vary from person to person.
The second component of the complaint is often implicit: "...and negative things are bad."
This is an excellent example of the equivocation fallacy. The equivocation fallacy is when you switch around the meanings of words. For example, let's say I use astronomy to show that stars are gigantic balls of hot gas. I go on to say, "Therefore, Tom Cruise is full of hot air." I've just switched around two meanings of "star"--those things we see in the sky, and those things we see on the screen. I also switched around different meanings of "hot air". In this fictional case with stars, I would probably fool no one with my reasoning, but in practice it is much harder to catch.
The word "Negative" has two meanings. In the first part of the complaint, it means focusing on criticizing claims rather than making new ones. A "negative claim" states what is false rather than what is true. In the second part of the complaint, "negative" means "bad". Just because these two meanings can be signified with the same word doesn't mean we should confuse the two. It's possible that the two meanings are equivalent, but you have to argue that fact.
So why might negative be bad?
- Negative claims practically advertise the things they criticize.
- Negative claims lead to disrespect.
- With what will people replace their previous beliefs?
- Negative claims can't unite anyone.
- There is something to be said about prioritizing claims that have wide acceptance. If we're criticizing a claim that is virtually unknown, then yes, maybe it is a little counterproductive. But religion and pseudoscience are hardly unknown.
- Sure, if a few people have their own self-worth all tied up with their beliefs, maybe it is disrespectful. But such people have already set themselves up for a fall, and deserve a dose of disillusionment. For everyone else, it is actually quite easy to respect the person while disagreeing with them. I do it all the time.
- Positive and negative claims are not mutually exclusive. You can criticize other claims while simultaneously putting forward better ones. But even without our help, it is a mistake to think that people will systematically replace their old beliefs with something worse.
- On the contrary, I think that negative claims often unite many more people. For example, "Atheist" is a largely negative label that only really says what people don't believe. But if you consider more positive labels like "Secular humanist" or "Objectivist", you've just split atheists into a bunch of smaller groups. These groups might be tighter knit, but at the cost of size. So which do you want: a group that loosely unites many people, or a group that tightly unites only a few people? Bad question. We can have both kinds of groups, and both play important roles. So there is no reason to outright reject negativity as a basis for uniting people.