Lightweight solar ovens are possible. I've seen some of them. [Does anyone know of some good designs, or which might be the lightest and most suitable ones for touring? I believe there are some plans on the web -- most of them are probably not so concerned about weight and size, but some of them might be.]
There are some limitations (like early morning, late afternoons and evenings, winters at northern latitudes, nighttime, shaded locations, clouds, etc.), but they can still be quite useful, and the fuel is freely given by the sun.
Cooking several meals at once might make sense for some people.
In certain areas of the world, or at certain times of the year, they are probably a very fine option.
[Maybe a solar cooker could be used to 'charge' a system for cooking another meal later on -- good afternoon sun could be harnessed to heat something to a very high temperature. This could be stored (well insulated), and then used for the evening meal.]
[Hot water could even be used in this way -- there is another recent thread about this.]
And there is something attractive about using the sun's rays to prepare food.