The purpose of the letters of credit in those regulations was to provide security to the plan instead of cash contributions that would otherwise have been required. It's not necessarily that the companies that avail themselves of this measure are on the verge of bankruptcy. It's really for corporations that would rather keep their cash resources for other purposes, to invest in the company and use the letter of credit as a security for the pension plan, so the pension plan is in effect in a neutral position, whether cash or the letter of credit is serving as security.
On April 29th, 2010. See this statement in context.