The purpose of SPs is scaling, IMO at least. WP are flat (doesnt matter of you are a lance or a battalion). Transport scales with force size, salaries, etc. Buying a single unit or increasing the skill of a single unit doesn't scale.
So if i were going to use WP only, I think I might make training something that scales. Spend x WP and roll once for each unit. 2-10, nothing. 11 improve the higher (worse) of P/G, 12 improve the lower (better) of P/G. Or something like that.
Perhaps make them 12 and 13 and give a +1 if the previous mission was won.