IMO, to spread or open the fingers is really dependent on what it is you are trying to accomplish. If I am going to strike, then they are closed - if I am not striking, then they are generally spread open.
Spreading the fingers helps to establish a 'live hand' in aiki arts, and as in all aiki, there is a mental and a physical component. The mental/intent/ki part has already been talked about. Aiki is all about making internal connections without muscle tension and that is accomplished via the fascia mostly - Spreading the fingers without muscle tension helps take the slack out the tendons, ligament, and fascia. Essentially, it helps you maintain a whole body connection where you can absorb and control energy from uke. Of course, this can be accomplished without spreading the fingers, but only after you have trained enough to have established a connected body, but your hands will be dead and useless for aiki in that case. One other note, any muscle tension used to keep the fingers open or closed will stop the flow of energy in the areas with tensioned muscle; this will inhibit a whole body connection and the development of aiki as well.
ps: I see a couple of post above came in while I was composing with some of my points already covered