It's one method used by Bitcoin to find out the IP of other nodes. There is a risk that the DNS seeds could co-operate together and attempt sybil you off the network by only giving you IP's controlled by them, but it is only one method used and it's only used during the initial set up of a node. The IP's are also sent over DNS, which gets cached by various DNS servers, making it nearly impossible to do that. After your node gets an initial list of node IPs your node keeps a DB of IP's and nodes share IP's between each other. The risk of an attack this way is absolutely tiny that its not even worth mentioning.