The documentation states that the library won't function properly if pin 10 isn't configured as an output, even if you aren't using it as the chipselect. I haven't been able to find any explanation as to why it won't function correctly or if it is just specific features that won't function correctly. Can someone explain what it is that won't work if you don't configure pin 10 as an output? I've commented that line out in some of the example sketches and it seems to still work fine. I am using a Leonardo with the sd card wired up to the spi pins and I'm using pin A4 as the chipselect.
It has to do with the internal SPI hardware inside the AVR chip, using that pin to configure itself to be in master or slave mode depending on the pin mode of that pin, output mode = master mode, input mode = slave mode. The AVR datasheet is the source of such gory details. If you were using a totally software drive SPI function then that would not be an issue, but you would not be able to obtain the high performance possible with the hardware SPI unit.