The point of the damage cap?

Bosses with a maximum hp of 2^31 - 1 hasn't turned out so well, though.

It's turned out fine due to PDRate/MDRate.

Also, dealing with signed ints makes a lot of calculations easier.

Code:

//Using HP as a signed int
if(HP - DamageTaken <= 0)
{
    PlayerDies();
}

versus...

Code:

//Using HP as an unsigned int
if(HP - DamageTaken == 0 || HP - DamageTaken > HP)
{
    PlayerDies();
}

Having to deal with the fact that the variable could overflow makes the calculations more complicated and more prone to error.