Drawback of refinement in a beam element

The beam element is already a somewhat high-order element. It's stiffness matrix is third order, if I recall. So it's already an accurate solution and tracks the correct curvature for applied end loads. Thus, you generally don't increase the accuracy of your solution by inadvertently refining beam element mesh for no reason. Nonetheless, there are times when you do need to add interior nodes to your beam spans as follows: (a) to apply interior point loads within the beam span, (b) to obtain displacement or stress results at interior points in your output, or (c) to improve the solution accuracy for beam span distributed applied loads.

What vonlueke said is right, some FEA or structure people say, that beam element-formulation is exact, it means the FEA solution should match the theoretical solution. Therefore the solution of one beam is in general the same as the solution of, say, 10 beams or as stated by vonlueke : "you generally don't increase the accuracy of your solution by inadvertently refining beam element mesh for no reason".
As addition to his comment about the need to add interior nodes to the beam spans, i.e. increasing the number of the beam, is also to attach beam(s) correctly to the adjacent elements. E.g. you have a plate modeled with 10 shell elements in its long direction and you will model stringers attached to the plate in long direction with beams, then you should model the stringer with 10 elements, so that the nodes of the beams will coincident with the nodes of the shell elements.
Hope this will contribute to your problem.

regards