What should you use 5.10 ...for what?

Coding?
- You get to use many improvements in projects that don't require backwards compatibility.
- You get to familiarize yourself with features you will eventually use yourself.
- You get to familiarize yourself with features that will be discussed here and elsewhere.

Testing?
- You can find out if your modules work for people using 5.10.

I had a few more reasons floating in my head, but they have escaped.