Mussolini was actually dead weight and a liability for the Germany as his miltary was very weak he took Ethiopia but his army was weak and outdated in fact he wanted to prevent war until 1942 for a variety of reasons so while Italy could be thorn in the German backside if he joins the allied powers the advantage would be short lived
Benny was more trouble than he was worth for Germany but he was ironically a big help for Japan, because the need to fight the Axis in the Med and North Africa meant that Britain couldn't adequately reinforce S.E. Asia to face the Japanese threat. Had he stayed neutral then most of OTL's Eighth Army and Desert Air Force would have been in Malaya along with better commanders meaning the Japanese lunge south would have been stopped significantly shortening the Pacific War.