This is one point of Raskin's design where the rationale is sound, but the solution he found was sub-optimal. Instead of a "pure" quasimode, modern interfaces do incremental searching following the search box style introduced by Firefox, with a persistent dedicated and non-obtrusive search field.
I find that this is a superior interaction, and it still can be considered non-modal. The style has spread to notifications too, with browsers showing small rectangles above and below the window instead of an obtrusive modal alert dialog.
It's so sad that the project died. The interface was very promising and I long for that keyboard I never used.
That book has been much more influential than it may seem - all the good interface designers seem to know it or at least follow its principles.
I would pay good money (a few hundred dollars, easily) for a mechanical keyboard that had those two additional keys under the space bar, as well as a vertical F-key cluster left of the main typing area, as seen on old IBM PC/AT keyboards.