Advantage of cup-like shape of blood cells, spores?
The functional implications of the biconcave shape of human red blood cells are not fully understood. Several hypotheses have been suggested as explanations, including bending energy (1) and effects on oxygen transport (2). More recently, it has been hypothesized that the shape is important for the flow properties of the blood cells in capillaries. Uzoigwe (3) states that 


  The deleterious nature of turbulent flow on the cardiovascular system
  suggests that the biconcave disc form has evolved out of a necessity
  to maximise laminar flow, minimise platelet scatter which in turn
  suppress atherogenic activity in the large vessels.(...) The biconcave
  profile of the discocyte means that much of the mass is distributed in
  the periphery. This increases the moment of inertia of the cell and
  subsequently renders the erythrocyte less prone to rotation during
  flow in the large vessels. Here it is suggest that this reduction in
  rotation promotes laminar flow and discourages platelet scattering by
  minimising the "Eddy currents" and it thus anti-atherogenic. A number
  of pathological mutations result in the red blood cell adopting a
  spherical shape as opposed to the biconcave disc profile. The sphere
  has a smaller moment of inertia when compared to the discocyte, as
  much of the mass is distributed round the centre. The spherocyte is
  hence much more prone to rotation during flow in the large vessels.


The advantages of various structures do depend on the scale of the system, as you say. For red blood cells, the size of capillaries and veins are of importance. The shape of red blood cells differ in animals that have blood transport systems with different structures than humans. Thus, the red blood cell shape is likely optimized with respect to movement in the human blood transport system, with oxygen diffusion also being a possible factor. To determine what advantages a specific shape of spores would give, you would need to consider the expected environment the spores would live in and their typical lifecycle.