It might be clear to some of you, but why is the LED13 driven by an OpAmp? In all examples, a LED is simply added with a resistor, but the official Arduino schematics show it with an OpAmp.
I've read some guesses: "using left over port on OpAmp", "power drainage"... but that doesn't help much.
So you don't load the pin. This is a dual use pin, you can use it for the LED or anything else. The SPI library uses it for the SCLK. The op amp makes sure you can put the full digital load on the pin.