Must I Use HSR's Nameservers for my Domains?

Of coure not. You can use any nameservers you like and simply make A records pointing to your account. The following are some of the advantages of using our nameservers: (1) If we need to change your IP address, we can do so seamlessly without service interruption; (2) If you decide to upgrade/downgrade your account, we can make any migration seamless without any work on your part; (3) Our nameservers are fast and well-maintained as they are a core part of our business, this is not true of everyone else and you may experience slowness or downtime if you select a separate DNS provider.