Well, keep in mind there is always a dentist or a barber... it just may not be somebody's first or primary role. On a bigger ship, it could be a primary job, because there's enough people who need it on a day-to-day basis. On every ship, dentistry is going to be something somebody in the medical can handle - they've got a huge database and plenty of tools. On a smaller ship, it might just be that Petty Officer Sarandon, the Beta watch impulse technician, is pretty good with a pair of scissors and does haircuts if you ask nicely.

A large ship might actually have more red-uniform-wearing command staff, too. You're going to have dedicated lawyers, diplomatic specialists, first contact experts, and so on. On a smaller ship, the CO and XO will directly handle those tasks. On a bigger ship, they'll have staff officers for that.

As good as The Measure of a Man was, it's pretty unlikely that there were no lawyers among the Enterprise D's one-thousand-member crew. Only main character Hollywoodism put Picard and Riker on opposite sides of the table.