I don't understand why everyone is so keen on being debt free in this game. Even when maxing out all loans it's virtually impossible to get a debt:equity ratio worse than 1:4, which is already pretty unrealistic for any company, let alone an airline. A debt:equity ratio of 4:1 is considered good for a real life airline guys, and those don't have the abstruse cash flows we get here. Paying back loans only constrains your growth, especially when done early. It's plainly stupid to pay the loans back asap.
To answer your question, an alliance gets points for every airline on every anniversary, and also for having a member airline in a scoring position in a certain stat.
You score if you are in the top 10% or top 20 airlines (whichever is more) of the following stats:
•On Time Performance
•Transported Passengers (YTD and Cumulative)
So, if you want to be of value for an alliance you should focus on attaining a good spot in those stats. There are however always alliances not interested in scoring and who will 'just' look at your personality.