Why should you hire a good attorney?
Good attorneys usually cost a lot more than average attorneys (and are obviously much more expensive than public defenders, who are free if you qualify for one). The money they will save you in the long-run, however, usually dwarfs the one-time upfront fee.
When you've been charged with a serious crime usually the last thing you want to do is rush out and spend a lot of money (unless you are unusually wealthy or a celebrity - in that case being charged with a crime might just be a rite of passage). For this reason most people facing crimes use their court-appointed public defender to represent them in court.
The problem with using a public defender as opposed to a private criminal defense attorney all comes back to the incentives that each attorney has in regards to your case.
Public Defender Incentive
A popular misconception is that public defenders are not good attorneys. This simply isn't true - they have usually gone to the same law schools. Public defenders do tend to be a bit younger, but that is just a reflection of the fact that they are trying to build up their credibility before striking out on their own.
Most public defenders, however, are severely overworked. Their case load is simply unmanageable and they are frequently defending the same people over and over. After a while the public defender realizes that in order to keep up with the workload they need to figure out how to wrap each case up as quickly as possible - and therein lies the problem. Their real incentive is to handle as many cases as they possibly can in one day (with a lack of emphasis on how well they actually defend each of the individuals represented by those cases).
Private Defense Attorney Incentive
Now consider the incentive that a private defense attorney has. First of all, good private attorneys make a lot of money - anywhere from $100k at the low-end to several million dollars a year for a high profile attorney. The difference between the attorney at the low-end and the high-end is their reputation - attorneys with good reputation can bill at higher hourly (or fixed) rates.
The only way to improve (or maintain) a good reputation is to provide top-notch defense to every single client. The good attorney will put all of their energy, work and connections they have in the industry to make sure that each client does as little time as possible (if they must do any time at all). Getting a few unnecessary convictions in a row or unusually long sentences can have a material impact on the rates they will be able to charge in the future. Their incentive is to always make sure they provide the best defense possible.