A lot of people use them for solving problems they DON'T have.
Lots of people use both Mongo and Node.js at ill-fated attempts at premature scaling (similar to premature optimization).
Also if node used some other language that wasnt a standard this might also be true. But again it uses javascript or coffeescript which is becoming more and more used all f the time. Even in mobile applications and gaming now with canvas. Obviously if you know python or ruby or php extremely well and can build whatever you need quickly with that then theres no reason not to. I was new to python so it wasnt a major switch for me.
How is scale "built in" to Node any more than any other modern language/runtime?
"I want to scale my app on modern, multi-core computers. I know, I'll write everything so that it all runs in a single thread!"
This is the same fallacy that gets repeated every time node is mentioned. If you're scaling up past a single machine you have to figure out how to share state between multiple machines anyway, so you may as well run a process per core. Then you have a single way to share state, rather than one way between cores and another between machines.
So unless you're writing a desktop app that needs to scale up but only to all of the cores of a single machine, you're sometimes better off simplifying the state-sharing logic.
And your 'cancer' language is unhelpful. What are you afraid of? That somebody somewhere is using a bad language when a better one is available? That the NodeConf organizers reserved all of the good hotel rooms?
Why are you so angry at a programming community?
It's not like "somebody somewhere is using a bad language when a better one is available" is not hurting CS in general.
I think the real danger is people thinking they've learned everything there is to learn and closing their minds. Which includes telling people that "node.js is cancer". Node is letting people do all kinds of interesting things, and it isn't killing off anybody's favorite language.
I'm thinking along these lines: http://www.dbdebunk.com/content2006.html
And that's --the second part-- is where they and you are wrong, as tons of teams have found out. Scale is never built in, you have to work for it.
"Scale built in" is another way of saying "Mongo is webscale".
When you first start out, you don't realize that flow control is going to be an issue, you don't realize that you're likely going to need to figure out how to do stream processing. Those aren't especially difficult to learn, but it's a delayed learning curve. You don't know that you'll need them until you run into a dead end. There are libraries/modules that help with that now, but it's not trivial to figure out which is needed (at least not for someone just getting started).
Node is an interesting tool, and I think it solves some issues elegantly. I think some people use it for projects it's not suited for, but that happens with every language/framework/tool.
You don't get scale from a tool. You get it from experience in knowing how to run scaleable systems, and architect scaleable algorithms / systems.