Occasionally, you see arguments online about why rolling 3d6 in order for ability scores is a a Bad Thing. Someone eventually points out that ability scores in the LBBs don't have as much of an impact as they do in later editions that have undergone "bonus inflation", but not everyone buys that argument; someone will counter that having a sub-optimal score in your preferred class's prime requisite will slow your advancement, so it prevents you from choosing the class you want. If you are unlucky enough to roll two ability scores below 6, you're screwed.
Does everyone think that's true?
Let's look at it from the extreme viewpoint: three classes, no ability score bonuses except for earned experience. We'll pick the "optimal" class based only on which one will advance faster.
I've pointed out that experience for the first five or so levels of Cleric and Magic-User has a simple mathematical relationship to Fighter experience: M-Us have to earn 25% more experience than Fighters to gain one level, and Clerics have to earn 25% less experience. I've expanded that relationship in Liber Zero and switched this to an earned xp bonus/penalty, just like the bonuses and penalties for high/low prime requisites, applying it to all levels.
If M-Us have a -25% earned xp penalty, even an Intelligence of 16+ only gives them a net -15% modifier, compared to a Fighter. That's better than a Strength 3 to 5 Fighter's advancement rate, but a Strength 6 Fighter is at -10% earned experience. A character with a Strength of 3 and an Intelligence of 13 or better would do about equally well as either a Fighter or a Magic-User -- so the Magic-User class is almost always sub-optimal for 1st through 5th levels.
In contrast, Clerics have a +25% earned xp bonus. Even a Wisdom 3 Cleric has a net +5% to earned xp, so it's better for a character with Strength below 13 to become a Cleric, and it's always better to chose Cleric over Magic-User, no matter what the scores. At Wisdom 6 or higher, the Cleric advances faster for the first five levels than any other class, no matter what the scores -- so the Cleric is almost always the more optimal class.
Throw in the Thief, with lower experience requirements that translate into a +40% bonus relative to Fighters. A Dexterity 3 Thief is always a better choice, xp-wise, than a Fighter or Magic-User, no matter what the scores, because the net bonus to earned xp is +20%, which can only be equaled by Fighter of Strength 16+. A Thief of Dex 6 is better than a Cleric with Wisdom below 16, and if a character has Dex 9, Thief is always the optimal choice, even if all the other scores are 18s.
What this means is that you can't just go by ability scores in an OD&D game. If you did, then counter-intuitively an Intelligence of 18 would rarely mean that you should choose the Magic-User class, and all characters with scores of average or better would be Thieves. Hardly anyone would play a Magic-User, and most characters with below-average Dex would be Clerics.
That's clearly not the typical make-up of an adventuring party. Most people play the class they prefer, rather than the class that would be "best".