Platforms like Node become a necessity as you'd rather be faster to market and be more agile vs. being caught out building something that is useless in a nicely scalable platform over a longer period. Lean startups FTW.
It's hard to say that a Node.js solution will get you to market faster than Ruby on Rails (where something as complex as user management is a drop-in). If anything, choosing Node would be the opposite of your advice.