The IRS wants to make sure the plan was created/started for the benefit of all employees and not created as a short term or temporary program to benefit owners or senior managers (think corporate or individual tax impact).
For 401(k)/profit sharing plans permanency is typically not an issue if the plan has been in existence for two years. For defined benefit plans (including cash balance plans), permanency is not an issue if the plan has been existence for ten years (although five years seems to be the industry rule of thumb).
Plans that may have issues with permanency must be able to show they had intended the plan was to be a permanent plan (as opposed to a short term or temporary reason). In addition, their must have a legitimate reason for terminating the plan including a business restructure, financial hardship, a change in the law impacting the plan, bankruptcy, merger, etc.
See Treasury Regulation 1.401-1(b)(2) for further information.