The
[channeling] condition indeed only works on channeled spells. This is to add the option to limit what you do so you don't interrupt a channel you're already in the progress of casting. There is no check to see if you're already casting another type of spell. Since the API function behind it is protected, no addon code can change its behavior.
An example of a macro using what [channeling] was meant for.
Code:
/cast [nochanneling] Barkskin
/cast Hurricane
This is a simple Druid Hurricane macro. Note Hurricane is a channeled spell. When chain casting, this ensures Barkskin is only cast when Hurricane isn't already in effect. Otherwise, casting Barkskin will interrupt the channel of a previous Hurricane.