You've got to either be stupid as you believe the fraudulent marketing rubbish that goes with their bikes (lest anyone disputes this check out some of their distributors' descriptions - complete bollox)
Or else you're not stupid and realise it won't make you any faster at all but have a ton of cash and want something with snob value / exclusivity.
I reckon there's a fair number of both among the Serrotta clientele.
Edit - there's then a third group who have been duped into believing they really need a custom frame.
I don't mean to me cruel, but there's a whole raft of local club riders that were all sold on custom frames by the same fitter whose prescription was remarkably consistent: "your top tube is an inch too long!". I cringe whenever I see this people, for the ass compressing fit they got, and the shrunk wallet.
No doubt some people would really benefit from a custom frame but it's nowhere near as many people as get sucked into it, that's for sure.
The explosion in off the peg choice over the last 5 or so years, especially in sportive type geos, has made the custom route look, for most, eccentric at best, and at worst a spectacular waste of money. I know they still bang on about bb heights etc, but it's got pretty lame.
Not sure how true it is, but I heard that the Serotta guys knew it was all BS, but that was their market so they stuck to it.
It's a lightly regulated industry in respect of marketing ... people claim all manner of crap to sell product.
Stiff, Light, Aero - Pick Three!!