Doesn't really change the issue of alliances at the core, but opens them to some exceptional abuse depending on the game stage. If you are facing off against an alliance in a normal game, then you are most likely out numbered as it is. Having one side in a war suddenly get double the attack potential, regardless of how strong everyone's economic bases are, can quickly become horribly unbalanced and even worse than it normally would be.
This is made worse if you do not put strong limits on how units can be used, or how often they can attack. What does it matter if you have the better economy to afford to build and field more units if a vastly inferior force still becomes an equal because they can just keep reusing the same unit to attack with while much of the 'superior' force sits and is unused?