Wessex, no offence intended. I'm in the inner city in Melbourne and we have our share of trouble as well.
I think you are making a connection between stiffer sentences and deterrence of crime that has not actually been shown to exist. Most drug users for example couldn't give a damn what the penalty for robbery and assault is, especially amphetamine users, who can be psychotic and totally off their tree when they commit offences. They just don't care - this is both from my brother the Magistrate as well as several police I know.
My brother has to think long and hard before he convicts and/or throws someone in the slammer because the "knock-on" effects for that individual and possibly his (her) family can be simply enormous and destructive for society as a whole.
Do it to a young first offender and you may be setting him up for a life of crime since prisons are the "Universities" in this respect and from what one hears there is very little rehabilitation work done in prisons these days.
For example, a conviction for assault in this particular case would probably trigger a "show cause" letter from a University Vice Chancellor, followed by a disciplinary hearing, sacking, and a complete loss of Academic career prospects with possible financial and personal consequences for her family if she has one. I would expect that her defence lawyer would have made exactly this argument.
On the other hand the young lady who told my brother to "get stuffed"(and other suggestions) in an assault case hearing was surprised when she got three months in the slammer and was lead away in tears. She expected to be back on the streets that afternoon.
I suggest "Light sentences" are something of an urban myth these days, as is the one most teenagers believe about their record being "secret" and getting a fresh start at age 18. I've had to explain to more than one teenager that once you have "form" you have it for life, and if you commit offences after age 18, your Judge WILL have access to your entire history and take it into account when sentencing.