If you can fit properly and are comfortable with a mass market frame, then that is probably the best option. If you don't fit properly on a mass market frame(ie. short legs and long torso/ long legs and short torso), then a custom builder can build a frame that fits you. Most buyers of custom frames are people that just want to have something different than everyone else.

I build my own frames because I'm the type of person that likes to build everything myself, not because I think its better than a mass market frame. Despite the hype, most frames are very similar in design and are just a place to hang the rest of the parts. The human body is very adaptable and you will get used to whatever you choose to ride.