Its not 5 MP. I mean "5%"! 5% less Armor Penalty as you can read above for every STR you have above the required STR.
With a heavy Melee buildthat go for say we 15 STR and a Steel Armor that needs only 8 STR you become 7x5% less armor Penalty thats 35%. That is a lot more Movement Points. And with some good Feats Nimble, Armor Slooping that can be reduced again so a Metal Armor,Sledge Hammer Build is more Mobile. Maybe to OP i dont know.
But that Higher STR let you use better Heavy Armors seems logic to mee.
Maybe a feat that adds that to the STR ability would be cool too. So the Player can choose what he like more. Can be called: "Walking Tank" lol.