You know how the royal family has to travel separately, in order to mitigate risk to the line of succession? Of course you do, it was the entire plot of King Ralph. Keeping your domain and hosting separate helps to manage the risk that your site can be hacked.
Think about it – if someone hacks your site, at least they can’t take your domain as well (this is assuming they figured out the account user name and password with the host/registrar). If they get your domain, that’s a little trickier, but at least your still have your site on the host. And if your host’s backup of your files fails, at least you have a copy of your site in your DropBox, your Google Drive (and for the extra paranoid, a hard copy of your site on a usb stick, which was a brilliant idea I got from an InMotion support dude. Not all heroes wear capes, guys)
You’ll also tend to feel kind of stuck if you want to transfer away from that host – and a lot of times those sweet first year hosting plans get pretty pricey the second year. It doesn’t cost any more to register somewhere other than your web host, and you can just pick one domain registrar and then shop around for a plan to host your sites. It’s a good habit to spread the risk around and make things harder for hackers.
Also consider two factor authentication and installing wordpress in a subdirectory. These are simple steps but hackers go for low hanging fruit, so harden up your site, yo.