I wouldn't really call them a new spin on taxis. They're more like the remises in Argentina, and unlicensed (and technically illegal) taxis in many other countries. Basically, you have the licensed and regulated taxis, where you have a relatively strong assurance that you'll get where you want to go for a controlled/metered rate, in a reasonably safe and well maintained vehicle, and if you have a problem you can write down the cab number and make a complaint to a regulator. You also pay a fairly hefty fee for all this.
If you're willing to take a bit more risk, you can flag down a remis, pay a couple pesos per person, and they'll take you from where you are to downtown, or from downtown back out to the residential area you live in. The drivers make these trips all day, fill the car as full of people as it can possibly be filled (they pick up additional people along the way until the car is completely full and then some). They run on the cheapest fuel possible (in Argentina, typically LPG), and are not necessarily well-maintained. So there's risk. And, while you typically get where you want to be OK, there's plenty of opportunity for an unscrupulous person to take advantage of individual riders (or even groups if they're organized well and coordinating with someone else). So again... it's a risk.
There's a reason taxi cabs are regulated as heavily as they are, and in general it's probably a good thing for public safety even though they're freakishly expensive.