"I have read that, but I also read that glue records where not the same as nameservers."
suggest that Glue records are
the same as nameservers.
"Is it really advisable to use own nameservers instead of digitaloceans`?"
It is not
advisable to use only one of your own servers as a nameserver, in lieu of the 3
nameservers that DigitalOcean allows you to use, for free.
"I mean, how serious is the loss you describe."
Is not being able to access your server a "serious" issue, for you? Just like many things in life, there are few one-size-fits-all approaches when it comes to computing. If you're simply using this server as a sandbox in which to learn the intricacies of server management, then I would submit that it's well worth effort to learn how to manage your own nameserver. If your droplet is a production server, you'd be nuts to only use 1 nameserver AND -- even worse -- host that DNS server on the same server as your site(s)/app.
As wonderful of a service that DigitalOcean is, hardware failures; network problems; software bugs; and more, are a reality of computing-life that no one -- not Google, Amazon Web Services, GoDaddy, Linode, RackSpace, etc. -- are immune to. In the IT world, redundancy has always
been a goal. It's just that for the longest time, it was cost-prohibitive. That is (only one of the big reasons) why DigitalOcean has been so successful, so fast: Now, you can have redundancy until your little heart is content, all the while not sacrificing quality and without breaking the Piggy Bank.
"Because, in case my registar does not support glue records..."
Perhaps it's time for a new registrar.
Learn how to create vanity or branded nameservers with your Droplets.