Wow -- It was cool to see they had parachutes on the plane itself. Is this common on smaller planes these days? I've always wondered if/why airliners couldn't do something like this.
Think of the extraordinary size and weight a chute or series of chutes would be for a short haul 737-sized aircraft. The cost of lugging those around across a fleet would be absolutely prohibitive. There's also little reason for this type of safety mechanism for such passenger jets. Commercial twin or quad engine planes are extraordinarily safe, and offer many more redundancies than a tiny single engine plane.
More importantly, though, the parachute is also meant to provide an escape mechanism for unrecoverable spins and stalls, not just engine failures or fuel exhaustion. In a spin or stall 'aiming for an empty patch' is, essentially, not possible, as the control surfaces of the plane are not usable due to reduced airflow. Additionally, in IMC (Instrument Meterological Conditions), it's not possible to see a safe landing spot. This is an ideal use case for a parachute, but if the plane breaks away as you say, the remainder of the plane will land in an unpredictable location.
There's also the increased complexity involved with ensuring all flight control and avionics connectors between the cabin and body of the plane break away reliably and quickly when the parachute is deployed, and never break away otherwise.
In short, adding a breakaway cabin might make the pilot safer (but might not, given the additional complexity for breakaway avionics and flight controls), but it puts more risk on the public at large when deployed, so it's understandable why such a system was never developed (with the possible notable exception of military ejection seats).
And as you say, they wouldn't add much safety because there isn't much more to be had. I can only think of two large jet crashes offhand that would have potentially benefitted (the DC-10 that crashed in Sioux City, and that Japanese 747 where the tail fell off) and that's from a period of decades.
Most airplane accidents happen on takeoff/landing or inadvertent terrain collision in bad weather. Neither of which a parachute would help.
Source: Pilot, skydiver aviation nerd.
Philip Greenspun is a pretty accomplished pilot and has some good writings on it: http://philip.greenspun.com/flying/cirrus-sr20
If his coining the 10th rule of programming wasn't enough for me to want to meet him add in the aviation stuff and he's definitely on my top list of people I'd want to meet.
Source: Pilot, skydiver, Cirrus owner.
I think these systems are banned in the UK as they're explosive devices and the risk of them going off on the ground was deemed more dangerous than their benefit. /noresearch