Here is why you have to play with in your bankroll and for the full buy-in:

Imagine you are at the 50nl table and you buy in for 10 dollars.

In the first hand, you pick up a pocket pair of threes and call a small raise.

With two players the flop comes 3AA and your opponenet with AK goes all in for his 50 dollars.

You have essentially lost 40 dollars on this play.

When you play, in the long run, your profit per hour depends on your getting full value for fortunate hands like this one. In essence, its virtually impossible to profit when you can't get full value for your big hands. Sure, you don't lose as much when you lose your whole stack, but the win more than makes up for the loss.