I agree it is a useful weed for wildlife. As previously stated, it is browsed - especially when young and tender. It's also a good quail plant. It would be a preferred weed in one of my "weed plots" but not where I'm trying to grow "food plot" species that I planted such as alfalfa, clover, chicory, etc....
The term "food plot" means different things to different people. To most, I think it means a place where they planted specific species of plants that they hope to thrive and provide nutrition to wildlife. I've personally never seen pigweed grow 10 feet tall like dogghr mentioned, but I have seen it grow tall enough to shade out typical food plot species, and I've seen it grow very thick like he mentioned to where it could actually provide fawning cover.
That picture above looks like a food plot to me, so I assumed the OP wanted it to be what "food plot" typically means when we use that term. I personally wouldn't be letting pigweed go to seed in mine, but to each his own view. Clover (and many other food plot plants) don't compete too well with 10 feet tall weeds thick enough to hide fawns.
In one of my "weed plots" that would be a different story. I would let it go to seed and then even lightly disk it to set back grass and make sure the weed seed got good soil contact. BTW - annual weeds have the same threat as faced by food plots - the invasion of sod forming plants - especially perennial grasses. So moral of the story is - decide what you want and carry out your plan. For pigweed, I have given you the plan both ways - to promote it if you want it and set it back if you don't.
Best wishes!