The trick is that one of those un-helpful suggestions is more dangerous than the other. If you consider them opposite sides of the same coin, that coin doesn't have a fair flip.
But thinking that running 12 miles a day is the only useful way to run, will discourage you from attempting to run.
To put it another way, saying "50% test coverage is fine, sheesh" should encourage more testing, because it lowers the bar you have to strive for to consider yourself to be "doing it right", and therefore makes it easier to talk yourself into trying at all.
Even better, what we're saying is "50% test coverage is a good start, keep writing good tests and if the number goes up, that's even better." Targeting 100% coverage simply motivates using the wrong incentive.
It is a subtle, but extremely important, distinction.
