Different data store types are used to solve different problems. An RDF store is designed to help solve the problem of ontological relationships. I wouldn't use an RDF store if I wanted to store information about who has the current high score on my video game or who has signed up to my site for a username and password.
Unfortunately some developers think that once you've chosen one particular storage technology (RDF, relational DB, document storage) that's the bucket you hae to put everything into in order to do anything in your application. I suspect that was the mental model of the other devs in the story above.
Well for one thing, with triple-stores it's not uncommon to expose an unsanitized read-only query engine (usually SPARQL), usually harmless because none of the data is secret. That goes out the window if you're actually storing business-sensitive stuff in there.
Aside from that, I guess there's theoretically nothing stopping you from using the triplestore for usernames/passwords (I hope you mean salted passwords) but sheesh, talk about killing a fly with a bazooka.
To be fair to those colleagues, it might have been less about them being clueless, and more about them wanting to offload work to my team, lol.
1) users often use the same username and password on various websites, despite this being dumb (because it's so convenient)
2) if you ask such a stupid question you probably won't be able to properly secure the machine hosting this
3) your users will be totally screwed when you get hacked
And this is why you should never store passwords but only a salted hash, and never trust any service that can email you your password when you click "I forgot my password".
What would you say to them?