I think the issue of how much sustained attention is ideal is highly dependent on the individual, the nature of that person's job and personal lifestyle, and the types of actions and projects on that person's plate. There are some people who do best in environments where they can focus on one thing for extended periods, others who do better switching gears a lot (I tend to fall into this category), or something in between.
Regardless of where one falls on that spectrum, however, I think we're at a point where for the overwhelming majority of us the nature of our work (both professionally and personally) no longer affords us the luxury of working on only one project at a time until it is concluded. Many of my projects require short activities followed by lengthier waits: a phone call to my lawyer to ask why I haven't seen the estate planning documents he promised, an email to a sales prospect asking her to clarify a requirements document she sent to me, a skype sent to my boss asking him about the status of a couple of things he promised me (these are all real examples, by the way). Regardless of how I feel about this, the genie is out of the bottle.
One of the reasons why I gravitated to GTD was that the system was geared toward the world as I recognize it: one in which we are required to juggle lots of balls simultaneously, and where those who lack a better coping mechanism will find themselves buffeted by the waves of the "latest and loudest" rather than steering the ship in a direction of their choosing.
In my experience, having a complete and truly clarified inventory of what has my attention gives me the ability to focus as appropriate, whether that means for 30 seconds while I leave a voicemail for someone, or for an entire day working on a complex sales proposal. Moreover, it enables me to take advantage of interruptions that represent vital opportunities (such as a phone call from a sales prospect who suddenly has an urgent need to buy something within a short period of time) without having my world unravel. After all, I know what other tasks are on my plate, and if I decide that the newest input takes precedence over predefined actions, my actions list will be there waiting for me when the time comes to get back to them.
I said earlier that I've been trying to be less dogmatic about GTD, but I think I missed the mark with earlier posts in this thread. The issue for me isn't that the "seven-minute" rule "contradicts" GTD, and that wasn't great phrasing. After all, one thing I've learned from reading this forum is that there are many people who report successfully incorporating things that are not "by the book" such as time blocking.
But my own experience with GTD has been that any time I've found myself having trouble focusing on the task at hand, it's generally been because I've done an inadequate job of capturing and clarifying individual tasks and projects that are rattling my cage (like a sink at home that needs to be snaking that I kept forgetting to write down til this morning), or a poor job filtering everything through my higher level horizons (for instance, there's a small scratch on my car that I had an action item to fix til I realized I just don't really care about a little scratch on my car, and that's OK).
I've tried things like linking tasks to areas of focus, priority flags, daily to-do lists and more and in my experience they've always been the equivalent of applying a band-aid when the bleeding is internal. If someone else's experience is different, I can certainly respect that and would encourage them to share their point of view. I'm not trying to argue for the superiority of my viewpoint, but instead I am sharing it in the hopes that someone, somewhere might find it of value.