biochemistry - What is the relationship between the light and dark reactions of photosynthesis?

My main concern is why are the dark reactions (those that produce hexose) needed? As far as I understand, the sole purpose of photosynthesis is to synthesize carbohydrates, which are only used for energy storage. So, why don’t plants store energy directly as ATP (by photophosphorylation) and not as carbohydrates? (Of course, that would be too bad for other creatures.)

Elaborating my question:

1. 
   
   

   Plants use chlorophyll to convert light energy into chemical energy of ATP and NADPH by photophosphorylation and chemiosmosis.

   
   


2. 
   

   Then plants use the ATP and NADPH to synthesize glucose by the Calvin cycle.

   
   

So, what is the need of step 2? Why don‘t plants just use that ATP and NADPH for their metabolism?

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PS: although @David has provided a nice answer to the current form of the question, the selected answer by @christian still remains selected since it answered my original query i.e. why excess ATP is not feasible for plants instead of why synthesis of carbohydrates is important. Any further answers/comments shall address the latter one, although none of them can be selected as correct since the latter query is not the original one.

Answer

ATP is a bad medium for storing energy as detailed here.

• ATP has a molecular weight of 507 Da


• Glucose has a molecular weight of 180 Da, and contains the same amount of energy as 31 ATP molecules