Why should you buy a custom cue? There are many fine production cues
on the market at almost any price you want to pay. Most of these cues are very playable and will improve your game over a cheap or bar cue. But these companies make so many cues that sometimes they don't have time to properly age the wood or select the perfect grain in a piece of exotic hardwood.

I think the best reason however to own a custom cue is a matter of pride. You will have a "one of a kind" and there will not be hundreds of people playing with a cue exactly like yours.

You should also be able to talk to the cuemaker and discuss the styles that you like and play a part in designing your own cue.