Balanced chemical equations not only provide a qualitative description of the rection, but a quantitative one too. It is not enough to know what the reactants and the products are, you must know how much of each is needed/produced.
Equation coefficients provide information on the relative number of moles for each species involved in the reaction, this they are so important.
If, like Meave said, something like this is given to you - magnesium chloride reacts with silver nitrate to give silver chloride and magnesium nitrate - you could still write a balanced chemical equation for this reaction if you know your way around a great deal of reactions.
So, if the question is -what is the mole ratio of magnesium chloride to magnesium nitrate? - you'd have to take that ratio from the balenced equation.
If the problem is more of a concept-type one, like -two compounds react to prroduce two other compounds. What is the mole ratio between the two reactants? - you can use the general equation form
Now it becomes easy to write mole ratios for all species involved
mole ratio of A to B
mole ratio of A to D
mole ratio of D to B
As a conclusion, you need the balanced chemical equation, or at least something like the general form of a reaction, in order to be able to say anything about mole ratios.