Yes one of the reasons Germany's spring offensive failed was that the attacking German units stopped to loot British supply depots. Not just for drink, but also decent food and boots. The contents of those depots also played a large part in convincing the ordinary German soldier that the war could not be won. It made the effects of the British blockade readily apparent as well as the ineffectiveness of the German Navy's submarine blockade. Their families at home were starving while the Allied troops were recieving luxuries that Germans hadn't seen since the outbreak of the war.
As for General Haig we have to remember what his men thought of him, not just the politicians and historians. His reportedly thought the world of him. They certainly turned out in their thousands when he died. Haig is also portrayed as a heartless butcher. Next rememberance day (if you're in Britain) buy a poppy and look at the black center. You'll find the words Haig Fund. His effort post war to assist the veterens is still doing it's job 98 years after the start of the Great War, this was not the action of a heartless brute. Haig, Jellico and Foch were the men that won the Great War and their portrayal as incompetants, butchers or both is completely unfounded. They made mistakes yes but they were facing a conflict unlike any they had trained for and had to learn on the job.
Without labouring the point it's also important to remember that Britain was alone among the major powers in not having compulsory military service prior to the war. Most French and Germans had done some military service before the war.
Also their senior generals were expected to command large formations in wartime. No British general had this experience.
Most junior British officers by 1916 were university graduates and white collar professionals who had no military training before August 1914.
What you had were generals from the fairly small professional army trying to organize a large enthusiastic crowd of amateurs into a fighting force capable of taking on a war machine that had been training and rehearsing for a full scale European War for 40 years.
Finally it's often forgotten that 78 British generals were killed in the war so the image of a Lord Meltchett sitting in his chateau is not entirely accurate. Also most British generals had seen combat before 1914 whereas hardly any Germans had experienced a battle. Haig, French, Kitchener etc had all been in battle and knew what fighting was like even if they hadn't seen anything like a WW1.