My two cents from a Lore/D&D perspective:
A specialization reflects a wizard's particular focus of study - the field of magic they spend the most time researching/practicing. Conjuration is one of those fields - but isn't impacted by them (with the exception of Evokers, of course). A Conjuration spell is a Conjuration spell, whether or not its caster is particularly gifted with Illusions - it still functions the same.
The divine censer is, obviously, an exception, where the caster allows their deity to direct where their conjuration comes from - but that's a god intervening, not something the caster is doing.
On top of this, the concept of Conjuration in Layo has been a contentious one in the past, and I can see all sorts of ways in which this could muddy the waters further. Using Necromancy as an example: Creating undead in Layonara is an incontrovertibly Evil act (barring GM-specific circumstances). A Necromancer who has a zombie as their Summon Creature I, however, could argue that they weren't creating undead, just summoning it from somewhere else and dismissing it when they were done.
On a mechanical level, the issue of balance would likely be troublesome as well, unless the differences were purely cosmetic.
(Also, Acacea is right - retrieving specialization requires NWNX using either LetoScript or the Exalt plugin.)