The reason that these two countries are classified as marginal states is because they are seen to be approaching the point where we would call them failed states. They have not yet reached that point, but they are clearly not stable.
Failed states have a number of characteristics. The two that are relevant in the cases you mention are:
- Lack of a legitimate government that has the recognized authority to act
- Lack of ability to provide some basic level of services to its people.
Both of these are close to being the case in the countries you mention. Zimbabwe's government is close to losing its legitimacy because of the unwillingness of Mugabe to truly share power with Tsvangirai. Haiti's government has been barely able to provide basic services since the earthquake.
Because these states are losing these abilities, but have not completely lost them, they may be classified as marginal states.