I know people who surfed the COBRA wave - ie, joined full-time for a company for a month every year or two to keep COBRA going and then going back to contract work in the interim.
Normally it just means you get to pay the full cost of your health plan, which is often double or more what you were paying as an employee. It's just so you're not immediately ineligible for the employer's group.
Unless yours is also paying the premium those 18 months, which I've never heard of but I suppose is possible.