The point is that when you add a domain to CloudFlare, they are scanning your DNS records and they are copying what they find (mail, ftp, smtp, etc A records).
If you have some customized DNS records (A records added in Directadmin) cloudflare will not find it and those records must be added manually.
If you want to offer this to your clients, they will get confused when the must add left DNS records in their CloudFlare account.
The purpose of a plugin for this it will be that the you can offer the clients the possibility to automatically create a account on CloudFlare and push all the records there. Also, the ability to see/edit the DNS zones, that are now hosted at CloudFlare directly from the DirectAdmin control panel.
CloudFlare offers an API (http://www.cloudflare.com/docs/host-api.html) that can be used for this and as far as I know cPanel, Parallels, Interworx and ISPsystem are already offering this.