I think there are at least a couple of entirely different aspects to all of this. One is to do with visibility, or feeling comfortable with being able to see and trust what you need to see. The other aspect is a desire for tentative pre-planning of what you will do in the near future.
Among the six types of dates mentioned earlier, I think the one that perhaps stands out the most is appointments. If we compare with, say, a tickler (first possible date) or a deadline (last possible date) an appointment has the very salient characteristic that you are busy during that particular time, more or less unavailable for other actions. You therefore must avoid double-booking that time slot. It is also usually characterized by one or other form of real-time “togetherness” with another person (phone, restaurant …).
A tickler or deadline normally can be without any or both of these characteristics. I just becomes “possible” (or ceases to be possible) on that day, but other than that the day has no special significance at all. You will not necessarily be busy with the action on that particular day; you can usually book appointments that day, and you do not want these dates to clutter your view when you want to schedule an appointment…
Most of us deal with this by keeping our appointments in a calendar, not in the list app. What Glass Planner does it allows you to also see the appointments in the project where they belong, and still see that these are appointments, not regular actions. I find this very inspiring and worth taking after. Some list managers allow you to export a calendar feed of your “dated” actions, but unfortunately they then dump all “dated” actions in that feed, which means you cannot see in the calendar which of these are appointments. And you cannot see any difference in the list either. It would be necessary to differentiate the appointments (calendar actions) from other forms of dated actions.
I think most of those of us who like GTD share the desire to avoid date planning. We probably avoid it for similar reasons. We know that things can change suddenly; we simply cannot trust such dates, and we do not have the time and energy to constantly reschedule everything. So many of us totally reject date planning. I do. But many of us, even I, still would not mind to be able to see some form of tentative “structure” (in some form) of our nearest future. Always having to look at a long list of possible next actions simply does not seem optimal.
GTD “micro-picking” of actions (by primarily looking at the context and intuitively also considering energy, time and priority) is definitely the way to go when we are in a given situation and want to make the most of it. For example, if we are at the hardware store, we will obviously do all the actions that can be done there, regardless of what energy we have, or the importance of each item. But when we look ahead a few days we often have much more choice. We may even decide in advance whether we will aim to go on an errand run soon, what shops to visit, and what things to buy. We just have not decided exactly what day, and we may still change our minds, but it is a reasonably firm tentative intention. The same kind of “reasonably firm intention” we can have for many other things, e.g. decide on certain contexts, projects, areas or specific actions that we will push ahead with very soon, and we may even have an idea of when or in what order.
In addition to the “situational micro-picking” GTD only only offers, at the weekly review level, the option to totally hide (inactivate) entire projects for the coming week. This is quite a brutal form of prioritization. It is either entirely “on” or “off”. GTD does not offer any guidance for how to pick certain selected actions or types of actions or contexts etc. No wonder that “preselection” or planning in the form of target dates, priorities etc etc still linger in people’s minds.
Yes, stringing things up on a timeline would be a way to tentatively get some form of structure - provided that it is possible to see which dates are just “soft” (tentative) and which ones are “hard”, and provided that the dates are easy enough to change, perhaps by changing the tentative date for a whole batch of actions in one operation.
In fact, I believe that “batching” would be a powerful feature, both with and without dates. By simply being able to see that a certain group of preselected actions (possibly from different projects and/or different contexts) will be done “together” in some sense we could reduce the number of main elements on our list. For example, we could pick a number of different bookkeeping and other actions that require similar energy and physical context and aim to get those done “together” in a few hours in one of the next few days or next week. A single soft target date or time slot for the batch as a whole could be optional, and would be easy to manage. And/or for some batches you would simply turn the “focus” on or off for the whole batch.