I am using a mixed system for ability scores:
First, the point buy. I use 15 points, but as always this is just a parameter. I put a limit of 8 (lower) and 16 (higher).
Second, the player chooses race, class, feats, skills, etc. It is, takes any decision to complete the character.
Third, every ability score is rolled again (3d6 in order). The higher score is taken, except if the point-buy score was lower than 10. In this case, if the rolled score is lower than the point-buy, you take the average score of both (round down). If int has been modified, choose the extra skill points (or delete ranks if int is lower).
Ex. you want a melee class so the point buy is allocated: 15 13 15 8 12 8. You choose a fighter, calculates hp, allocates skill ranks, etc.
Then you roll 6 times 3d6 in order: 12 8 18 12 6 6. You final scores are: 15 (max[15,12]) 13 (max[13,8] 18 (max[15,18] 12 (max [8,12] 12 (max[12,6] 7 (avg[8,6]). As int is now higher, choose extra skills.
- People chooses professions for a variety of reasons: personal tastes, social pressures, family, etc.
- People don't maximize, they even don't know their ability scores except for a stimation. I mean, they don't always take the profession they are best fitted to.
- In fantasy tales and in real world, there are charismatic warriors and strong wizards (well maybe there are no RL wizards).
- People change, especially at puberty/youth, when many have already choosen their profession.
- The final point buy value (I use score of 5 is -6 points and a score of 6 is -5 points) is 22 as average when using the standar array. BUT it is not allocated as you want. About 6% will have point-buy total value value lower than 15. About 12% wil have point-buy total value higher than 30. About 46% wil have point-buy tota value of 19-23.
- The 15 point buy means that you char is viable. You may have some really low scores (as low as 5), but they are not in the scores your class really needs. You also may have 18 in all your abilities, but this is as probable as if you roll 3d6 6 times. Even a single 18 is only attained is you roll that with 3d6 (1/216).
- Optimization and min-maxing. Some players may want to use a point-buy distribution of 16 14 14 9 8 8, hopìng that they will roll at least average scores for those lower scores. The average point value would be 23.5. That's little gain, and they may find they are playing a 5 wis fighter or wizard. The 16 cap in scores for the point buy means they are not able to buy the 18 score: they have to roll for it.