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The placeholder markup is included in Creole to keep so called "memento" in the source, so that engines can "mask" complicated objects in the source code.
The idea was that the engine would identify in unique way any objects that cannot by represented in Creole, either by enumerating them or by generating some kind of a hash. Then, in the edited Creole code, the objects would be replaced with <<identifier>>, so that the users can move them around or delete without knowing their markup.
So far, no implementation uses this. The markup has been sometimes used to represent macros, plugins, various additional elements -- but each time in a slightly different way.
The proposition is to remove the placeholder markup from the Creole specification, and move it into the recommendations instead -- the wiki engines can choose whatever mechanism and markup is apropriate for them, because the additional objects are not transferable between wikis anyways.
Instead, we can try and come up with a recommendation for a common macro/code/plugin markup -- so that there is a chance of compatibility between engines, without actually restricting the creativity of their authors.
moved discussion to Talk.RemovePlaceholderProposal