I am developing a Socket.IO-backed real-time application in Node.JS that will be used by couple of hundred users simultaneously at any given moment, and I need to store some basic details about each connected client (and remove those details upon client disconnections).

I have read that using a key-value store (such as Redis) is the preferred choice for storing such data.

Why is storing data in a regular in-app variable (object, e.g. var connectedClientsData = {}) bad compared to storing data in a key-value store such as Redis?

Is it only to support scaling (e.g. multiple NodeJS-based application servers can connect to a single, central key-value store), or are there some more serious drawbacks?

There are a couple of issues at play:

1) Yes, scaling. Not just to multiple servers, but also to multiple CPUs via something like "cluster" (which you can find on npm).

2) V8 has memory limitations currently based on its browser-based heritage. They are tweakable to some extent, and being removed more thoroughly in the next version of V8, but it's still something to consider.

Also you're much more likely to restart your node process (to update your application) than you are to restart redis, thus losing all the sessions.