They do call it "Billion Dollar Baby's Gear!"
In all seriousness, there's a market for everything, and the buggies do have some really good features. $500-$700 is actually the entry range, for the tier of strollers these models play in.
They are competing with the brands such as Bugaboo, UppaBaby, Quinny, and Stokke.
The manufacturers build these strollers with additional features and functionality to justify a sticker price above the normal umbrella stroller you're gonna get for free with a $50 purchase at ToysForYou stores.
The premium tiers offer features you're not going to get from most Graco or PegPergo models, such as the bassinet, the adjustable toddler seats, higher weight capacity (extending the useful life, because I certainly don't always feel like carrying my 100%+ 2 1/2 year old!,) the ability to easily convert into a dual (or sometimes even triple) model, superior design (telescoping handles, transformer like folding, washability, crazy easy maneuverability) lightweight-but-still durable materials, built in rainguards, sunshades and bug netting, etc...
Are all these features "must haves?" No, but at the same time, if it makes life easier, why automatically dismiss it? (We *could* wash our clothes or dishes by hand still, since those are conveniences not necessities, but we don't normally deny ourselves the use of those expensive tools.)
I guess what I'm saying is, if it fits ones budget and meets ones needs, then it's cool that they consider or purchase it.