There is no real point, just means what? Spending a lot more for one extra boost which unless you do it for at least the number of wars they have already won changes nothing overall.
It is easy as it is now you join the faction that fits your budget and desire if you want everything and are prepared to spend you join FUN, if you are ok missing a boost a month and want to spend less you join RK or SB. If you are happy to miss a couple of boosts per month and spend a lot less you join a top 10 faction etc.