The reason you cannot cook frozen dinners in a toaster oven is the toaster oven heats the frozen food up too fast so that it does not cook it all the way through.
So part of the frozen dinners can be raw and cold while the outside of the food could be cooked.
You can however cook frozen dinners in the toaster oven once you thaw them out first and put them in a toaster oven tray.
A toaster oven is cheaper to use than a regular full size oven.
The toaster oven uses only 1500 watts at most while a full sized electric oven can use as much as 4000 to 5000 watts of electricity per hour.
So if you only need to bake or cook something small then use of a toaster oven would be much cheaper than turning on the full sized electric oven.
However if you do have a natural gas or propane stove and oven then the full size oven would be cheaper to use because propane and natural gas are cheaper fuels than electricity.
I use my electric toaster oven for cooking small foods and small servings but for larger servings of food then the full sized oven is much more efficient, faster and cheaper.
You can cook chicken, pot pies, small pies, cookies etc in the toaster oven and it works fine when you only need to cook a small amount.
Larger amounts of food are best cooked in the full sized oven.