Shows how strongly experience informs assumption. Your Belgian experience leads you to think that every uninjured pro takes part in his national event everywhere every year; my experience in Britain makes me think that unless it is a course and a date that suits him, no pro anywhere bothers with these events.
It used to be that way in Denmark.
I think, especially for smaller countries (in terms of number of WT riders) where quality can vary greatly from generation to generation, it very much depends on the competition.
When you have a generation of winners, they all want it. They want to ride in a "special" jersey, all season, that they have taken against hot competition, while they contest the biggest races in the world.
When you have a leaner generation, the prestige drops, as it is often won by some random domestique, who happens to be one of the few good enough to get a job on the WT.
Back in the first golden age of Danish road cycling, with Riis, Soerensen, etc., the Dannebrog jersey got coveted, after Riis rode the Tour and won wearing it. For a few years they all wanted in.
Then followed some lean years, where nobody really cared much.
Now, with the emergence of the Asgreen, Mads P, Kragh generation, it has become hot property again.