There is, in fact, a reason for this payload/padding within RFC 6520
From the document:
The user can use the new HeartbeatRequest message, which has to be answered by the peer with a HeartbeartResponse immediately. To perform PMTU discovery, HeartbeatRequest messages containing padding can be used as probe packets, as described in [RFC4821].
>In particular, after a number of retransmissions without receiving a corresponding HeartbeatResponse message having the expected payload, the DTLS connection SHOULD be terminated.
>When a HeartbeatRequest message is received and sending a HeartbeatResponse is not prohibited as described elsewhere in this document, the receiver MUST send a corresponding HeartbeatResponse message carrying an exact copy of the payload of the received HeartbeatRequest.
If a received HeartbeatResponse message does not contain the expected payload, the message MUST be discarded silently. If it does contain the expected payload, the retransmission timer MUST be stopped.
Credit to pwg at HackerNews. There is a good and relevant discussion there as well.