J.K. never hints at a lawyer profession, per se, but that doesn't mean there isn't one. Most of the trials in the books are under special circumstances.
For instance, Harry's was while he was under age, and it is common for minors to have simple hearings rather than lawyer-ridden trials.
Now that I think of it, none of the others are trials. Sirius was sent to prison without one. Of the scenes that Harry witness in the pensive, two were simply reading of the verdict and sentencing (for Bellatrix's bunch and for Bagman). The other (with Karkaroff) was after hie was tried and convicted; he was simply trying to get into a plea bargain, which would not have required a lawyer at that stage. The lack of lawyers at any of these events isn't indicative of the system as a whole.
In the case of the pensive scenes, there may have been special circumstances still in effect from the War, as often happens. Even if that wasn't true, there was nothing stating that there weren't lawyers present; they simply never were needed during the scenes we saw.
All in all, I would say that if you want magical lawyers, you would be justified in having them.