A couple days ago, I posted about Strength damage bonuses and mentioned in passing that I'm against fixed ability bonuses in general. This raised a question about why I'm against ability bonuses. I've talked about different elements of my opposition in several posts, but it's worth it to bundle up a summary of my reasons.
Keeping Things Manageable: If you have several sources of bonuses to rolls, especially if those bonuses *stack*, you basically have multiple things to check, multiple numbers to tally up, before you get to the actual attack roll. My motto is: the fewer potential sources for bonuses, the better.
My basic attack roll formula is a variant of Target 20: if d20 + hit dice + defender's AC is greater than or equal to 20, the attack is a potentially deadly hit. I add the best weapon bonus to that, and the best armor bonus to the 20 target number (no stacking from multiple sources.) I also add 1 or 2 points to the side with the advantage. That's pretty much it, although maybe there's some rare exception. That reduces the handling time immensely
Keeping Things Dangerous: The reason some people love a +/- 3 range of bonuses from ability scores is, of course, to greatly improve their odds in some situations. They want to be able to easily eliminate some challenges without risk. By keeping bonuses low in general, you maintain the risk, even across many levels.
Keeping Focus on Action: Which is more important: what characters do, or what's on the character sheet? The fun, for me, does not come from finding the best character build to reduce risk, but to accept the risk as given and plan around it, purchasing the proper supplies, gathering information, testing things carefully, taking time during exploration. That means focusing on the fictional situation and the actions you take to deal with it, instead of focusing on a bunch of numbers.
If you don't want players to focus on numbers, you have to keep the numbers off the character sheet. I don't mind players looking at a list of what they are carrying or what they know, but I don't want them adding up bonuses for various options to make the best abstract strategy choice.
De-Emphasizing the Scores: If a small change in a score can make a significant change in survivability, players will fear rolling a "helpless" character. If few of the scores have a direct effect on survivability, then players will be more willing to accept lower scores, or a wider range of scores. This means more variability for character types.
As JDJarvis put it, "the rules work better when they support a large spread of scores and encourage player tactics over dice rolls from months before."