I think this is one of those situations where you can "break" the rules only after you know them. I think the reason for keeping your calendar "sacred" is the same reason you don't put errands on your @Calls list. Doing so would undermine the effectiveness of both lists because your brain knows something is amiss and expends energy inefficiently trying to make sense of everything. In other words, it becomes "stuff" again, in David Allen's terms. So too with the calendar. If you can't look at your calendar and know what's "happening," you spend a lot of energy trying to make sense of the mess, figuring out what's important, what's not, and so you stop trusting and using your calendar because it just stresses you out.
So that's the reason to only put things on the calendar that belong on the calendar. And it's the reason to only put things in your kitchen drawer that belong in your kitchen drawer, and to put socks where socks go, and to put batteries where batteries go. David Allen says it is so gratifying to have everything in its place not only because we get our psychological and physical arms around the problem, "but because there is nothing of that nature lurking anywhere else" (Making It All Work, p. 201).
I believe this outcome is attainable while still blocking time on your calendar. The rule to me for what goes on your calendar is not what but rather of what nature? So long as your calendar is for the kinds of things you commit to do at a certain time/place, there's no reason you can't have an appointment with yourself. The problem is when people don't treat it that way. I guess we don't respect ourselves as much as we do our colleagues. That's a problem of self-management and integrity in my view, it's not fatal to the calendar in and of itself.
I think a sophisticated person finds a way to block off time for important but not urgent tasks. I learned that from Stephen Covey. It doesn't have to be on the calendar and it doesn't have to be time-specific, but it could be, and it actually might be highly beneficial. Me personally? Lately I've been keeping a list on a small piece of paper with each day of the week and 1-2 important goals for that day. I schedule all my meetings after 1pm and try not to look at email until after that time, so I keep my mornings protected for my most important work.
My point is this: It is useful to set aside time to work on important things and let the little stuff fill in the negative space (the opposite of how we usually try to get things done). This could be accomplished a number of ways, but I don't think it's anti-GTD to use your calendar so long as you understand what goes on the calendar and what doesn't.
(Side note just so I don't make any assumptions: Blocking time on the calendar isn't an alternative to parking those reminders on your Next Actions lists. At least not how I've come to understand things. The appointment isn't the "Next Action," it's a period of time in which you will DO the next action. Also, if you didn't have these things on your lists, where would you pull these important but not urgent tasks from?")