First of all, the advantages of a high morale early in the game or during war are clear (population grows/regrows faster, etc.)
In addition, some 60 - 65% morale is needed to ensure that your political party stays in power, whether in war times or in peace.
But what if your planets are fully populated and you're not at war, what would then be the advantages of lowering your tax to increase your population's morale above those 60 - 65%?
A happier population is supposed to mean that more people will pay taxes, but in which timeframe (e.g. 5 weeks, 10 weeks, ...) does this pay off? Decreasing your taxes will decrease your income immediately, so as of which point will the increase in taxpayers offset the lower tax rate?