My friend interned at the FAA 20 years ago. He said the norm there was to write "Concur without comment." I thought that was brilliant. Of course, when I use it in conversation, no one gets my reference and thinks I am weird. But that is going to happen anyway.
I usually shorten it to simply "I concur", if no comment follows, it is a given. Maybe if you are using a lossy message format where a potential comment might get left out it would be necessary to make that part clear, but for most things it is a given.
