From: South Carolina, USA
Keep in mind that you can see on the counter of the HQ if it has 1 or more actioncards available!
Showing the action cards on a HQ is a nice feature, but it doesn't help if the HQ is off the screen (like higher level HQ frequently are) or they aren't on the top of a stack. And the method to play the cards is still cumbersome as Ctaardvark said.
I read an article once on how to design software and and it talked about verbs and nouns. It is hard to explain in just a few words, but one example given was of a way to save phone numbers on phones. One system required you to hunt down the save functionality, a verb, to save a number and the other let you save a number, a noun, when the number was on the screen at that time. The noun method put forward as the superior design choice and a more natural way for the user to use the system. I wish I could find the article I am talking about but it was a few years ago I read it and I couldn't find it on Google.
I view playing the action cards as the verb and the unit as the noun. I have a unit I might want to play an action card on so I should be able to do it from the unit itself instead of having to hunt down the way to play it.
This explanation probably makes no sense to anybody but me, but the bottom line is the method I described would, I think, be a much more natural way to play action cards.