I understand that many people like to spin and I know I could benefit on two bikes or two ratios (high and low) from all the spinning.

I just feel like if you're going to ride a fixed gear, why not run the highest ratio you can safely slow down on as well the highest you can handle? It only makes you stronger, which logically, will make you faster. At the very least, you'll be able to run higher gearings after a while and then just spin at a comfortable cadence while bombing down hills going ludicrous speed. And your bike will look cooler.