In some counties, prosecution offices have policies whereby they offer the same plea bargain for each instance of a particular crime. For example, the prosecution may routinely offer a couple days in jail, probation (with attendant conditions, like alcohol abuse treatment), and specified fines (in addition to mandatory license suspension) for first-time drunk-driving offenders who weren’t driving with an excessive blood-alcohol level. In that kind of situation, is it worth spending money on a private lawyer?
Sometimes it might be; other times it might not. First, it's risky to rely on secondhand information indicating that the prosecution always makes the same offer for a particular charge. (That's one reason why you might want to at least pay for a bit of consulting time from a lawyer experienced with the local system and its players.) Second, if there’s any chance that you will fight your conviction, whether through a pretrial pleading like a motion to suppress evidence or by going to trial, you should definitely go with counsel (whether the public defender, if you qualify, or a private attorney). Third, a lawyer may be able to convince prosecutors to give a better offer than the standard deal, or (occasionally) convince them to drop charges.
If you’re dead to rights, a lawyer may not be able to get you a better plea than the standard offer. Paying a reasonable price for a qualified lawyer is rarely a waste, though. At a minimum, an experienced lawyer can take a lot of hassle out of the process and guide you through whatever requirements the court imposes on you. That lawyer might also give you helpful information about clearing up your record down the road, perhaps with record expungement. Lastly, you might not know whether you have a viable defense if you don't at least consult with a defense attorney.