[nl]

It's not really a PaaS, because it only provides a single service (in this case realtime messaging).

That doesn't mean this model doesn't make a lot of sense.

If you are using IaaS then it can be a lot quicker to use something like this than build it yourself. Same for things like Search, or (sometimes) Database services or distribution (think CDNs).

Even if you are using a PaaS it might still make sense to use this (depending on what your PaaS supplies).