Just wondering what's in it for them.
Well, there's no such thing as a free lunch ... but running a DNS service is relatively cheap (low maintenance, doesn't need fancy servers, doesn't use a huge amount of bandwidth). And since its free, they won't be providing you with any SLA/SLG anyway.
So I suspect you'll find the basic service is genuinely free, but will be subject to various constraints carefully designed to encourage you to upgrade to the paid service.
I've never used a DDNS service because I've always had static IPs on any connection I've had anything to do with at work or at home. It just makes life so much easier.
What are you planning to do with your DDNS anyway ?
If you're planning to run a mail-server, don't bother unless your ISP is OK with it and your IP is not listed on Spamhaus PBL (most ISPs with half a brain block inbound mail to non-approved ranges and post their non-MTA ranges on Spamhaus PBL .... both done for very good reason).
If you're after remote access to your computer, there may be other options that don't require opening stuff inbound.