What if it's beer he doesn't like?
The problem is you are going to only be drinking 3 gallons of beer.
Or perhaps using the batch to step up yeast to be harvested for another batch (I do this with lagers - 2 liquid vials of lager yeast for a 2.5 gallon batch, then use the yeast cake in the next 5 gallon batch and then that yeast cake in a 10 gallon batch).
Of course big beers would not be a good idea for this step process based on yeast stress, but there are times when it makes sense (especially like when you are making a beer that is really experimental).
But if you end up really liking the beer, then, yes, that is a problem only having 3 gallons...