There is also the fact that in this case the Terrans can fight the Minbari on their own level. Its not Earth being curb stomped for 2 years before being they bother to attack the homeworld. Earth can and does hurt them. Badly. Basically if they join on earth's side, not only are they replaying a debt but they're tipping the scales and have the potental to reap the benefits of victory both technologically and in relation to their gains.