It's beneficial in the way that with a single press, you can remove the files from your phone while still have access to them in the case you'd need more space for whatever reason (be it videos or photos etc). You won't be tied to using a computer for your removing and adding, just a Wifi (or data if you don't have a stupidly low cap) connection.
If you have a sufficiently large storage pool, say a 64 or 128 GB microSDXC, you don't need extra space and you won't be removing files from your phone. Life is simplest when you simply never remove files, or only remove them rarely.
For large files, say 15 GB of vacation videos, you won't be syncing that shit to the cloud anyways. You'll want them on your computer to edit them or watch them in the first place, which means you'll be connecting to a computer anyways. Plus I don't really want to sync 28 GB of music to the cloud, the initial upload would take like a week.
More storage is always good, yes. But in the world we live in right now where 16GB still is the starting point, there are ways to circumvent without having to spend extra cash (even though SD cards are "cheap," not everyone can afford that) just so you can store your lifetime's worth of music which you'll probably never listen to.
16GB is only the starting point on Moto and Nexus. Everybody else has moved to 32GB as the base. Maybe if you ever considered a non-Nexus device you would realize this, but I know you worship at the altar of stock Android so oh well.
The current playlist I keep in rotation has about 300 songs on it. I actually do listen to the music in my collection. Maybe not all 28 GB of it at once, but I'm not one of those people who keeps 1 song on repeat for 4 hours.
SD cards are incredibly cheap, you can get a 64GB microSDXC for $40. And that's an investment you carry over every time you upgrade your device, as long as your next phone also has a microSD slot.
