I think there are two possible reasons for an airline to offer a cadet programme:
1. They can't get enough direct entry applicants who meet their standards, or
2. Cadets are cheaper: can be put on a lower wage; pay for their own training; can be bonded for a number of years; etc.
Both QF and JQ have offered cadet schemes over the years, for whatever reason. If I had to guess, based on respective business models, QF have done it for reason 1, and JQ do it for reason 2.
Cadet programmes typically accept around 1 in 50-100 applicants. This means they get to choose carefully, and assuming their selection process is sufficiently rigorous, are not selecting "silver spooners", rather the best people for the job.
Of course the selection process is not always perfect, and while I've seen plenty of excellent cadets, I've seen a few disasters as well. But the same goes for those who join direct entry, whether they be ex-GA or ex-military.
The other interesting difference is that QF cadets go into the back seat. JQ cadets go into the RHS. If I was a JQ Captain, I would hope that I was on top of my game with a 200 hr pilot in the RHS.