I for one like having it provide a source of Metamagic mana, and more importantly grant Channeling III to unitclass_adept units onMove. That means your adepts gain xp faster and have slightly better odds of not be resisted, and more importantly that all your Mages can learn Archmage spells. (You may still want to upgrade to Archmages for the free promotions based on how much mana you have though.)
Also, in my version the Tower of Mastery and the Tower of Necromancy combine to be insanely overpowered. I changed Lichdom to be castable only by living units with Death III in the city with the Tower of Necromancy, but removed the python block only letting you cast it when the unitclass isn't maxed out. I removed UNITCLASS_LICH and made Liches an Archmage UU, so they can't be used to get around the limit on the number of caster's anymore unless you can get non-archmage units to get Death III. (Hmm...I just realized I hadn't played the Amurites lately...) More importantly though, my Liches start with Combat I-III, Extension I, Potency, Vile Touch, and are Immortals (intrinsically, not through the immortal promotion). Having the Tower of Mastery and Necromancy in one city mean an essentially infinite supply of very powerful, immortal casters.
(I'm starting to think I need to put the python block back. I think there is actually a call that counts the number of a specific unit instead of unitclass, so I guess I'll use that. I may also need to only let living units use the Cave Trials (in my version the Cave of the Ancestor's lose their normal effect, and instead grant +1 sage slot plus much more importantly the ability for any arcane unit to cast this spell that will randomly either kill the unit or let it pick a free promotion. The chances of the unit dieing are pretty low of Adepts, but get higher for stronger casters. There is also a smell change that a Channeling III caster could get the Caswallawn promotion, granting +1 affinity for all mana but being unable to cast the spell again and being instantly killed if another unit manages to get the promotion.) spell, as Liches can who fail the trial are resurrected and can just try again the next turn.