Double the training gains at half the value. Plus, do you plan on spending 2:1 on stamina? Probably not. But do you plan on spending 4:1 on agility? Yup! So training agility at 5% literally is saving you 4 skill points you can spend somewhere else.. somewhere more valuable than 2 stamina, for example!!
Training % isn't value. Value is how you play on the field. Just ask which you'd rather have..
1 agility or 2 stamina
Or another question to ask yourself
would you spend 4 skill points for 2 stamina?
Answering these questions will lead you to realize the REAL value of training. Just because you train for +2 stamina at 52 doesn't mean it is "worth" 4 skill points unless you would have spent those skill points there given the chance.