There's maths behind RDBMS - the relational algebra and calculus. These give you enormous power when you don't know how you want to query your data in future (something you'd think the "agile" crowd would be all over). But instead, the prevalent thinking is "just stuff it all in and worry about it later". Experienced DB hands call this "painting yourself into a corner".

Part of the reason though is it is easier to get an idea of what you need from a NoSQL database and what you'd be giving up if you are already on an RDBMS, right?

I think the advice is right on and I see NoSQL databases as very much useful adjuncts to the relational systems, not as replacements.