This opinnion, unless a humoristic entry, would be greatly ignorant of how evolution works. Plants produce hard cellulose walls and any number of nasty chemistry for a simple purpose: they are trying to make it harder for herbivors to eat and to digest them. There is, of course, a co-evolution going on between plants and herbivors. By large the plants are winning.  We are the perfect example of just that: we have essentially given up to compete on that level of the game and are have resolved to eating the herbivors that haven't quit the plant battle, yet.

Absolute energy efficiency is, in most eco-systems, not a problem or we would have seen plants with photochemical mechanisms far superior to photosynthesis. After all... a silicon solar cell beats photosynthesis by a factor of 20! But it seems to be harder for evolution to switch from the local extremum that photosynthesis represents to a totally different chemical electron transfer and storage mechnism than to keep improving other survival mechanisms (like toxins). As long as everyone competes using the same photosynthesis engine, there is very little selection pressure from that side.

As a result... we can not expect to get much more energy out of plants than we already are (except that we don't use most of it efficiently). We could get two orders of magnitude more energy out of solar cells, though.