Yes, females can throw eggs without males....not what I would actually call spawning though (Doing the nasty

). It typically still takes a triggering event though such as a change in water chemistry/temperature.
Most "show people" will always turn to the females for the reasons stated...imposing appearance of a large female and there is no doubt (at least for me) that females and males should not be kept together past the age of 2-3 (to prevent spawning and ultimately damaging a prized show Koi). Also, females typically develope slower and "finish out" at a larger size than males, also making them have typically a longer period of enjoyment.
BUT, for those with smaller ponds that enjoy competing in the smaller sizes, a male Koi/pond can be pure enjoyment as well. Also, males will tend to be cheaper (for the stute anyway) within a given "quality range". Males can also have their place with the non showing hobbyists due to the lack of size (typically) and easier care.
Just my $.02
Steve