With the LTE speed of 70mbps, you could use up your 6GB in 11 minutes.
It's not about how long it takes to run a download of a media file, you're completely missing the point. It's about the drastically improved latency and responsiveness in downloading complex modern web pages, streaming, and caching stuff.
For example, The Verge homepage is almost as big as a song. If the question becomes do you need to make every page load .5 seconds instead of 2 seconds then "yes" wouldn't seem so ridiculous.
Why would I do that kind of sustained downloading on my phone? That's what the server & home internet connection is for. LTE on the phone isn't for blowing through 6GB In 11 minutes, it's for grabbing the data you need as quickly as possible.