One of several reasons would be geometry. Splicing two arms together can be done but the issue is what is happening to the geometry. A different spindle will definitely alter the geometry to some degree and I think this is what Robert is more concerned with. That would be my concern also, simply replacing one spindle with another does not optimize the inner control arm points to work with the new spindle upper, lower and tie rod ball stud points and this can throw bump steer, camber gain and roll center out the window.
Robert, I have measured quite a few spindles just for this reason when I am designing a suspension around off the shelf parts. I just finished measuring an aftermarket spindle for a project I am working on and it isn't all that complicated. You do have to have the parts though, so it does commit you to buying (or borrowing) them.
I will post some pics of the spindle jigs I built for this project which lets me set the spindle up at ride height (really not necessary, but handy) to measure all of the requisite points to develope a 3D CAD model of the spindle. I usually do this as a very rough model because the points are what is important and as long as the shape is the correct envelope size I can confirm clearances to other components.