I have a very dear friend who is a man of faith - a Mormon. I enjoy talking to him about religion because he's secure enough in his beliefs to have conversations with me about the issue without getting too upset. We're not trying to change each others mind, we are swapping philosophies and at times challenging the logic behind what the other person believes.
A little personal history before we get on with this. I was raised in a small town in Washington from the time I was in fifth grade to when I graduated high school. After I moved into the area, a Mormon temple was built and Mormons started moving in which gave the local economy a boost and raised the rosters at the local schools quite a bit, causing a need for more schools to be built which created jobs as well. The Mormon immigration was not a bad thing - but you would have thought it was the end of civilization as we knew it. I remember reading about people being singled out and abused because they were Mormons in local shops and at public events and my dad talking about it, saying we're all first hand witnesses to a case of Christians eating one another. It was a lesson I learned about the importance of doctrine to religious people. To this day I can't understand how people rationalize doctrine as divine in any way, but it was eye opening to see at such a young age groups of people who believed in the same god and the same messiah but still stubbornly hated one another because of differences in details.
All this being said, I generally liked the Mormon kids that moved in. They were down to earth, humble, smart, and most of all - they knew that their religion was a whole lot of stuff that to an outsider wounded like complete woo. They understood and Mormonism made sense to Mormons because they were Mormons. Some of them didn't believe it, some of them did. Most of them were like my friend - they generally followed the religion but they also had their own opinions and they didn't fear any repercussions for deviating from strict adherence to the church.
My friend and I were talking about atheism and he brought up that tired old argument that atheism is a religion. I feel like this argument is a derisive one - to tell an atheist that what they believe is religious is like telling a vegetarian that their lifestyle perpetuates the meat industry. I didn't feel like my friend was trying to mean, though, I felt like he probably heard this argument himself and not being terribly big on critical thinking he absorbed it and accepted it without really questioning it.
Dawkins (was it Dawkins who coined this phrase? I'm pretty sure it was...) was spot on when he said that atheism is a religion the same way that not collecting stamps is a hobby. The assertion that atheism is a religion is ridiculous and not really worth exploring in and of itself. I explained to my friend why atheism isn't a religion and he agreed eventually with some reluctance. His argument ended up hinging on conflating the definition of religion to the point where not only atheism, but the belief or non-belief in anything would be considered a religion. Of course, if the only way you can logically prove an assertion is by creating a personal definition of what something is, you've really only convinced yourself - the one person who doesn't require convincing.
I asked him after the atheism is a religion thing was past, how it made him feel when people called the Mormon religion a cult. He shrugged, told me it didn't really bother him that much because he knew they were just being mean. It was then that I told him that I knew how he felt.
If Mormonism can somehow be considered a cult or not, it doesn't matter. The connotations of the term 'cult' are highly negative and so when you apply it to Mormons or any group of people, you're using the term as an insult while also being able to claim that you're properly applying an apt term to a group. It's the same thing when you insist that atheism is a religion. Atheists make no secret of their general contempt for religion. It's true that some atheists don't care too much about religions, but a good deal of atheists seem to view religion with feelings that range from a mild contempt to a passionate distrust with a myriad of emotions in between, all of which could be considered negative. This is not a fact that only atheists are aware of, this is well known, and so to say that atheism is a religion is a way to insult atheists without openly insulting them. It's a low blow that doesn't have to be answered for which is the worst kind of insult you can throw at someone.