Depends what you keep in it and how long you want to store food for.

The recommended maximum temperature of - 18 deg C is the highest safe temperature for foods that are particularly vulnerable to spoiling, like seafood and meat, when stored for the maximum recommended period. If you only want to keep something like ice cream in it then you can increase the temperature to around - 14 deg C, the same goes for frozen vegetables, and probably bread.

If you do increase the temperature above - 18 deg C then storage times will be significantly reduced, but if you can, for example, tolerate a maximum seafood storage time of, say, a couple of weeks, then even a temperature as high as - 12 to -10 deg C will probably be OK.

I suspect that for many people a really cold freezer isn't needed, as many will consume the contents within two or three weeks. For those that grow a lot of their own food and freeze it then long storage times (and hence lower temperatures) are probably needed.

One problem is that people tend to be ill-disciplined when it comes to checking how long things have been in a freezer, so increasing the temperature is likely to lead to an increased risk of consuming spoiled food.