True. But remember in the episode with the Victorian Christmas/Bird in the lab that Leonard accuses Howard of preferring Raj to Leonard because Howard can denigrate Raj. And Howard immediately demonstrates it by the way he talks down to Raj.
They ALL do it to each other for various reasons to varying degrees. Leonard insults Sheldon regularly, usually with justification.
It does remind me of an interesting point one of the head writers for The Simpson's said in an interview - that they NEVER use 'put down' humor. Most comedies do this - and so do people - especially men - in Real Life.
There has been a lot of interesting research into male friendships in recent years. Here's an interesting article in the NYT: This article is interesting because her brother was 'broken' as a boy - as were both Sheldon and Leonard and Howard. Raj, not so much. In fact, Leonard tells Prof Proton that Sheldon is broken and that he, Leonard likes to feel needed. It's an interesting dynamic among them all.
This article presents a bit of what TBBT might look like without the advent of the women: "Men Sharing Rent & Friendships for 18 Years."
Of course, there are exceptions. Oddly enough, the naval veterans group in which I am active [submarines] use put-down humor with each other regularly - but also exhibit the traits popular culture associates with women's friendships: hugging, saying "I love you", visiting when ill, engaging in hobbies together, and so forth. I guess they got the machismo stuff out of the way while younger.