[Turin]

It´s easy to argue against him. A spectre costs 10 death gems, which is worth at least 300 gold to me.
A caelum seraph costs 100 gold and is just as good at researching. So factoring upkeep in it takes 30 turns until the spectre is cheaper than the seraph. That´s a pretty long time, so you are usually better off using national mages for research.
And if you find a library it´s a nobrainer to buy sages.