>useful if you want to do more than poach food at 120c

Slow cooking almost always gives you excellent results with vegetables, meat or fish. There is already a market of crokpots, sous vide and steamers, that I wouldn't dismiss as "poach food".

And for things like a pizza or crispiness in general, that need higher temperatures, I doubt you can get decent results with a microwave.

The usual 180°C / 350°F set has more to do with convenience (highest temperatures that doesn't burn the outside before the core is cooked) for cooking things as quick as possible, than tastiness of the food.