I guess that makes sense. That way you can have 3 archmage-shades, and keep one slot open for your other units to wane and be used as great specialists.
After reading xienwolf's post, I think I'll make shades immobile arcane units. (To prevent the AI from getting their unit stuck in the field, I'll probably make the spell only castable in a city.) They wouldn't have any of their own promotions (including channeling), so non arcane units that wane won't get any arcane abilities, but archmages will keep on learning. This will probably be done together with making them national units. 4 archmages, 4 archmage-shades, and maybe Liches (although since the Sidar abhor the undead, I wonder if maybe they shouldn't be allowed liches. Maybe Shade and Lich could share a unitclass if you don't let them cast Lichdom? Of course, with the changes I plan to make to Liches that might be inconvenient for me) sounds pretty good, even if 4 of them are stuck in a city