Morrus said:Why is he dead at -10 hit points and not at -9? Why does he hit if he rolls a 28 but not a 27?
It's a game, and it uses numbers; there's got to be boundaries somewhere.
This particular boundary does not need to exist at all.
A PC has 50 hit points. He should be able to do Second Wind anywhere from 1 hit point to 49 hit points. If he has an ability, he should be able to use it.
An arbitrary "can only do this at half or less hit points" rule is an additional unnecessary rule. The 4E designers are claiming that they are trying to simplify the game. Fine. Remove rules that do not need to be there at all. IMO.
Can you come up with a reason for why this rule NEEDS to exist?
The only reason for it is because some game designer thought of it.