As has been suggested, one of the central aims of the revolution was to avoid the vices of Mr. Jones and humanity. The animals wanted to be true to their own nature, if we can put it that way, and in doing this they were to reject all the false trappings, comforts, and hypocrisies of mankind.
In drinking alchohol and spending their time in leisure, the pigs have separated themselves both from the other animals and the ideals of the revolution.
With the consumption of alcohol, the pigs have come full circle. One of the biggest problems with Jones was that he was a drunk and would forget to care for the animals and the farm. When the pigs drink alcohol, they are carrying on that tradition and becoming just like Jones, only worse because they are subjugating their own kind.
It is another step in Napoleon's move toward complete control of the animals and the farm, and by drinking the alcohol, he shows that he believes he and the other pigs are above the rules that apply to the other animals. The consumption of alcohol is another way that the pigs can distance themselves from the working animals--and the booze is another luxury meant only for the elite animals of the farm.
One of the first things that the animals did in the revolution was to destroy Jones's beer keg. He was often very drunk, and in fact was passed out asleep when the revolution began. The animals thus seem to have associated alcohol with Jones's laziness and abusiveness. Commandment #5 stipulated that no animals would drink alcohol, and the pigs' indulgence in drinking demonstrates how corrupted the principles of Animal Farm have become.
It does this because it is simply one more way in which the pigs are moving away from the principles of animalism. The pigs have gradually been moving more toward being like the humans. The fact that they are now drinking alcohol just shows how far they have moved towards abandoning their earlier principles.