The behaviour which you have pointed out is expected, because you are setting the value of the cell array element A{1} to 2, but not setting a value to the symbol. Assigning a 'double' value to a 'sym' variable would just convert the 'sym' variable to a 'double'.
You can use the function SUBS to substitute a symbolic variable with a 'double' in MATLAB.
Here is some sample code to do the same:
>> syms x,y;
>> A{1} = x; A{2} = y;
>> subs(A{1}+A{2},A{1},5);
You would get the result to be
y + 5
