I think the reasoning was that you should get better at the abilities you use most i.e. str for fighters, int for wizards, wis for clerics etc. and I agree with this to a point. One of the frustrating things for me in 1e and 2e was that your stats never got any better no matter how much you exercised, read, prayed or whatever. Unless you found one of those rare magic items (or a wish, etc), your abilities never increased. I loved it when they changed this with 3e, but I think they went too far. I like the percentage increase of stats every level. I think hack master does something similar.
Here's my idea:
I think every level you should get a percentage reward that you can divide into any number, or just one, of your stats. When you reach 00 you gain a full point increase, your percentage resets to zero and you make the adjustments. The percentages would have no bearing on your stats otherwise, they would just be a tracking mechanism. This way if you want to focus on one stat you may. (I imagine a lot of players would.) But if you want to try to raise more than one, go for it. This would also put the raising firmly in the DMs hands by allowing them to decide how much to give out. You can give the recommended number or more or less as you see fit.
It's not nessesarily a new or novel idea but it helps to see it spelled out.
Thoughts?