I think about all the people who have unusual body proportions who can't afford custom bikes. They don't need custom bikes. They get by just fine.

Therefore, you only need one if you can afford it.

A good analogy for the need/want off-the-rack/custom bike frame conundrum is buying a good men's suit and wearing it on a fairly regular basis (for those of us who still have to do that from time to time - if you wear a suit once every tem years, just rent the damn thing). For the vast majority of men, it is possible, and not even that hard, to buy a suit off-the-rack that will fit well, look and feel good and last a decent amount of time. It will satisfy all your suitly needs and almost all youor suitly wants.

There are a small percentage of men whose body types are such that buying off the rack is just not an option unless you really don't mind looking and feeling kind of funny. For them, a custom tailor is the only way to go.

That does not mean a custom tailor is wasted on the "off the rack: guy. A custom fits that little bit better, often in ways you don't consciously notice - you just know that it is somehow more comfortable, everything hangs just a little bit better - it just feels "right," that is when you notice it at all. Is it more expensive? Of course it is. Is the extra cost worth it? YMMV. All I know is the handful of people I know riding customs built for them just kind of sigh contentedly and get a goofy frim whenever they ride them.