We probably wouldn't burn wood directly. Ethanol is also biomass. In fact, every single fuel we "burn" is biomass. The hard part is getting hold of carbon that isn't glued to some oxygen already. Once you have it, you can process it into whatever form best suits you.
You have to be careful when you speak of "efficiency". You have to specify, with respect to what expended resource, and what desired goal? If we say the expended resource is "land area" and the desired goal is "powering all of civilization", then indeed the the "efficiency" of growing trees and burning them is not so great, compared to say solar panels. But if the expended resource is "environmental damage" and the desired goal is "heating a cabin in the woods", burning tree wood is extremely efficient.
You have to be careful when you speak of "efficiency". You have to specify, with respect to what expended resource, and what desired goal? If we say the expended resource is "land area" and the desired goal is "powering all of civilization", then indeed the the "efficiency" of growing trees and burning them is not so great, compared to say solar panels. But if the expended resource is "environmental damage" and the desired goal is "heating a cabin in the woods", burning tree wood is extremely efficient.