Players from other countries can participate in county season in England and usually players refine their skills in these matches but other countries will not accept foreign players in their domestic season and it is a fact that playing in counties are an opportunity for the associate players.
What happened to Dirk Nannes is a perfect example. He got selected for Australia, and played one or two matches for them. And as per the ICC rules, someone who represents a test nation needs to wait for 4-years before he becomes eligible to play for the associate nation again. And there ended the international career of Nannes. I would rather blame the stupid rules created by the ICC. When Nannes was selected for Australia, there was no need for him to wait for even one day. But once he was dropped, he could no longer go back to the Netherlands. The same happened with Boyd Rankin of Ireland and Gavin Hamilton of Scotland as well. They played one or two matches for England, and thereby became ineligible to play for their respective nations.