Muddy has said my thoughts extremely well. It's bugged me for years that their sacrifice was utterly meaningless. Their action served no delay, and they should have retreated sooner. Their leadership should have authorized such a retreat almost immediately. But as it stands, the efforts of the ground troops didn't even leave a dark spot on AT-AT armor, and thus they continued to give their lives and resources (which, as we know, are very finite compared to the Imperials'), and made no difference. Better to live and fight another day than pointlessly waste life and ammo.