If taking Debt for no reason was a thing, then you could take it even in a Base only game--or in an Alchemy-only game, to counter Possession (which is the only reason I can think of to randomly take Debt).
Being able to take debt voluntarily to counter Possession would be
awesome! It would both weaken a controversial and powerful card, and lead to interesting strategic decisions.
By my reckoning, if you quantify the expected benefit to you at this point in the game of taking a turn, and call that
b, that means it's worth
b to play Possession on you. If you deliberately took a debt of
b before your turn then you lose
b if you're not Possessed, and your opponent gets no benefit from Possessing you, so they won't.
If you take a debt of
b/2, however, your opponent has a choice between Possessing you (benefit of
b/2 to you by paying off your debt, residual benefit of
b/2 to them) or not. You've turned Possessing you into a break-even decision.
But your opponent might not be in a position to play Possession. Say the probability of it being possible next turn is
p, I think you should take a debt of
pb/2.
This ignores all sorts of side-issues and complications, but illustrates the kind of consideration that would arise.