You know actually... if you look at CN right now you can see exactly why there should be a seperate action channel.
What action shows are on weekdays?
Transformers/B Daman and Teen Titans in the 6AM spot
One Piece twice a day (Miguzi and prime time)
GX twice a day (both in Miguzi)
Code LYOKO in Miguzi
DBZ in prime time
And the occasional Ben 10 and Naruto after Fridays
Sundays have even less when there isn't a Ben 10 marathon... Pretty much just TMNT and 9:30 Ben 10.
Leaving Saturday as the only day that has a significant amount of action programing. With TMNT at 6AM, some action in the Saturday block and Toonami in prime time.
Just giving Zatch Bell and Naruto weekdays at this point along with what they already have would suffice for me but it doesn't seem CN is willing to give over more than an hour to acquisitions in prime time and since Ben 10 doesn't have enough episodes yet and Teen Titans is all but dead to CN along with the other action originals that came before it there isn't an original action show to take a prominice in prime time. Though JLU would be perfect for that at this point I don't believe they will ever air it in prime time. Probably The Batman will air in prime time sometime this year if it comes back and the ratings don't suck.
So with only that much action total on CN I think an all action network is perfectly reasonable. CN has a ton of action shows it could be airing but wont make space for and that's the main reason such a channel has a lot of potential.