i asked this same question a long ass time ago. and got little to no feedback. ( and the feedback i did get was re-tarded )
in the proper pox nora i envision split themed runes would always be split and never ever tied to full faction. especially things like stitched arise which are literally unusable outside of their theme. but thats not how it is. there really have been many times that i could've built my stitched bgs slightly better. but because of many of stitched themed runes being ff fw or ff ud that i was forced to waste valuable rune slots leaving the bgs unoptimized. at this point i think things like stitched arise becoming split will never happen.
I think the point is that the two stitched themed spells - Arise and Disintegrate - are in different factions to promote UD/FW stitched bg's over FF stitched bg's (or UD/X or FW/X splits). It enforces the intended split, and the only time it hurts your BG is when you try anything except UD/FW.
Actually its a lot less true with kanen, because only idol of kain taris is SP-only. Pack savagery and scythe of gore are both split SL/SP.
Cultural contamination doesnt really count as a kanen racial spell though, because it only gives negative abilities to kanens. It's really a moga spell.
I was pointing that out as a split spell. It's been so long since I played Moga that I still thought that G'hernbound added stats when they were near a G'hern.