It is good that people are aware of the risk, but in reality the risk is quite low. It is standard procedure for a lineman to ground a line he's about to work on, to mitigate the risk of any power source feeding it. But I'd never advocate a DIY setup. But damn near everyone I know with a portable generator has a suicide cord and just flips the mains. Ah well.
How many linemen are killed a year in the US due to backup generators or similar? Are there no procedures they can do to work on lines safer?
But since lives are at stake here, we prefer to have layers of protection rather than just one thing. Grounding jumpers can fail if not properly connected (a large mechanical force will be applied to the jumper when the phase-to-ground fault current flows through it).
Bottom line, respect the electrical code and connect generators through a transfer switch. These rules are in place for a reason.
In fact it's likely the transformer on the street between your house and the network that's responsible for the death of the linemen. IIRC that's a step down transformer (so step up when run backwards like this.)
oh, you mean like solar PV on-grid setups ...
Some battery systems, like the Tesla Powerwall include a contactor that allow their microgrid battery systems to conform to the standard without a loss of power to the site they power.
Needless to say, if you're looking for a backup power source this isn't a property you want.
(an idiot who the state of NM certified last year to install his own on-grid solar PV system, so doubly an idiot!)
A correct install is of course also required as it's possible to screw up and unintentionally cripple this important safety feature.