One advantage to the current implementation is that it gets you uniformity of behavior regardless of whether you are in portrait or landscape mode. I do the double tap to get a new action in a project by muscle memory. Without that, depending on orientation, I have to move my hand to a completely different part of the screen (or use the other hand), and that would certainly break up the rhythm when entering tasks in quick succession.
Sure, viewed in isolation, it would be slightly quicker if I didn't have to make that extra tap to specify that the action was going to the current project, but the emphasis is on
slightly. I took all of my remaining actions, stripped out the large collection of actions that were programmatically generated (email from Omni support, Curt's template script, raw URLs, etc.) and ran the rest through Mathematica to get a histogram of length:
Median length is 29 characters. Yes, I checked, there really is a 188 character action name, and as it happens, I did type it in by hand on the iPad! Even if we assume the projects all had contexts set, no dates needed to be set, and whatever other hand-waving one might do to make the assumption that the total number of taps required is very closely approximated by the length of the action name, that extra tap is clearly in the noise if you believe in action names that are descriptive. I'm not counting all of the extra taps needed for capitalization, numbers, symbols, either.
Grocery shopping I do with repeating actions for staple items, so there's actually rather little typing to be done there. If I strip off the "Buy " prefix I put on actions where I am to purchase something, leaving only the name of the item, and just look at purchases:
That extra tap is still in the low single digits as a percentage.
What I think would bring real value in this sort of situation is the ability to copy and paste multiple lines as multiple actions. If you've got an existing list somewhere, you don't want to retype it, and you don't want to copy and paste each line separately. If you don't have an existing list, but want to make one, the ability to copy the list via copy and paste and make multiple actions would allow you to use your text editing app of choice to make that list. Much more powerful!