from a multiplayer perspective definitely, as humans are capable of very complex diplomacy not bound to the in-game mechanics
from a singleplayer perspective, you will probably be able to affect some changes. for example Italy isnt the leader of the axis but i imagine italy could spend its time supporting fascism in states that it thinks would be of great benefit to the axis. it'll be an impact, not as big as the faction leader (esp in my example, as Germany gets bonus political power which just means they can meddle with the world even more than italy can if they were on an even playing field otherwise).
although allies minors will have a lot less to do, i don't see much of a point of say, Australia engaging in diplomacy because all of its neighbours with the exception of new zealand are guaranteed to join the allies and be under attack by the japanese. Same for say Canada.
although should the situation allow you will be able to meddle with international issues, like sending volunteers to China still (assuming china still exists by the time the allies can send volunteers)