Kingposts are, almost invariably, in a lot less tension than the bottom chord of a king post truss.
Per the article if TF 73, the bottom chord of the Lynnfield truss carries 16,600 lbs of tension, while the king post only 4,200 lbs. This shows the great efficiency in the truss, as it is much easier to let the wood do the work (continuous bottom chord) than any sort of joinery, to carry the tension.
Trying to manage that amount of load (numbers will change based on your application) with traditional joinery, or even a spline, seems quite optimistic. Couple that with the tendency for the bottom chord to want to bend at the king post connection, and you run into other issues as well.
