I don't use Dropbox, nor am I a Node dev. Why would a node_modules directory tank Dropbox?
Because NPM's first operation is to pull in half the internet as modules so you can write 'Hello World' on your webpage. Thats too many files for the Dropbox client to handle.
Does it not suggest that node_modules in the source directory isn't a good idea to begin with?
The project directory would contain everything needed for that project including node_modules (which is generated from the package.json)
Every nodejs project is structured this way... node_modules is always sitting next to your src...
It is usually git-ignored, which means it isn't part of the "real" source directory (the one that is shared with other devs). It's a temporary output of the build process, in essence.