It depends on which setting and what kind of story you have your characters play.
I play in an historical setting and in a fantasy one. Both are home-made.
I the historical setting, a player who is a knight remains a knight all along : he has a small land to lord over, peasants to protect, taxes to raise, he must deal with justice and must obey the feudal code. A craftsman has a trade, soemtimes a shop, probably a wife and some kids. At some point in their lives, something happen that will band together the knight, the craftsman, a sergeant and an herbalist. But they never stop being what they are : the knight must act like one, the craftsman can't really stop his trade (or his family will starve) and the herbalist must keep being one, she does not know any other trade, how would she survive ?
In the fantasy setting, I'll just give the example of the current party. The elven bard is here to note down and later sing the tale of the party. She also is here to gather inforamtions and learn hidden secrets. She's been asked so by her queen. Trus, she adventures underground, but the reason why she is there is her trade. She is not interestied in money, if she was, she'd be singing love songs for a rich human lady, in a warm and cozy manor, she would so not risk life or limb into a labyrinth of hard stone populated by goblins, undead and "things that can't be named".
The dwarves ? One is a master smith, he is here to reclaim an ld mine taht turned out to be full of dangers. What he wants is the silver ore that will make him and his companions rich. He wants the mine. (HE made me start the campaign, actually).
It quickly appeared that somehing was terribly wrong here. It started when he met dwarve refugees from the mine, he promised to help them if they agreed to make him a king. THey agreed after much debate. NOw, our darf needed a solid party to take on the cahllenge.
First, the bard. Because she is a good friend, because she is in a strong position at the elven court. Because she knows magic, because she fights damn well for a "light-bone". And the bard had so many reason to accept it was not even a difficulty to convince her.
Because a king, a dwarven king at that, must have a saga that sings his feats through eternity.
Second, the priest. No sane dwarf would embark into a fight against the unknown without a stout dwarven priest of Gormaz. His job is to have the gods side with the would-be king, to raise the spirits of the refugees and to counter the gods of darkness (understand : any god that do not like dwarves or condone alcoholic beverages). It would suffice the priest, but he also is interested in salvaging holy relics or stone tablets and maybe restoring a former place of worship.
Third, you need a scout. Let's not be short sighted, make it two. One for surface (one never knows). An elven outrider will do : she is a good friend of the bard, keeps to herself like any elf should do and is a deadly shot. On top of that, she knows things that grow or walk like nobody ! Why is she here ? Religious reasons. She is a devotee, some would say a fanatic, to the goddess of life, fertilty and birth. Undeads and other "things" are prey for her. She does not know squat about underground fighting but it was not sufficient to deter her from accomplishing her "holy mission".
One for underground : like this sneaky fellow, the dwarven rogue. A woodcutter and carpenter specialized in mine architecture, he also is a shady character who likes to go unnoticed. A talent some dwarves find precious in underground warfare. Of course, hs knowledge of caves and mines is precious. Needless to say, the words "lost" "mine" "silver" and "recovery" put together in the same sentence lighted a fire in his eyes that yet has to calm down.
Than, last but not least. You need a professionnal of underground warfare. Here comes the Dwarven fighter. All steel for tip to toe, a large round shield strapped to his back, an impressive array of weapons at hand, he fears nothing. He is a killer, the kind of guy with muscle, determination and brains. Not only an excellent warrior, he also is a brilliant tactic and strategic mind. Why is he here ? Because it's his duty.
All along he story, each character must use his knowledge, abilites, and skills to gain new parts of the mines, reach the underground city, keep control of what they conquered and put it back into service.
So, if you just have stories for kill/hide/investigate, indeed, you don't need professions. The problem (if there is one) does not lie in the system, it lies in the stories the GM proposes to his players.
