Spirulina like any food or supplement when properly handled, processed, and used can be a useful supplement to your existing diet. It is very dense in both calories and protein per gram compared to other foods and contains almost all of the amino acids your body needs to function properly as well as several key vitamins and minerals.
Like any food try to identify a safe source for your food that is respected and trusted. Even a simple and common vegetable can kill you if improperly handled and processed.
In nutrition as in all things there is one important rule, all things in moderation. Per tablet you are right there is little nutrition there. but per its total wight it is actually very dense in nutrition when compared to other foods. A single tablet of soy, beans hamburger, apple or any other food also has very little nutrition. But pound for pound a pound of Spirulina has more nutrition than a pound of almost any other food. Not that I suggest eating a pound of it as I said all things in moderation but you get the idea. As little as 10 grams a day (a handful of tablets) could be benificial.
Don't try to live off Spirulina alone but if your diet is elsewise poor it might help to supplement some missing components. It is very promising in the fight to end world hunger and malnutrition when combined with other cheap mass produced crops such as Soy, Rice and Beans.