Imperfect evidence is still evidence, and there are possible worlds where this test will yield useful results.  They may not be ready to defeat truthtelling spells right now.

"Okay, let's work under the assumption it's an honesty spell.  Does not prevent attempts at deception, does not enforce objective truth.  Um -"

Keltham isn't comfortable with this.

...people's lives and money are at stake, somewhere in the background, he really should do it anyways.

"I'm very sorry about even asking this, but I'm in very weird circumstances where I know very little about this world, so - are you alright with asking me one or two questions about this whole situation, as it is known to you, while you're still under a truth spell and there's been very little time for anybody to prepare for that?  I'll try to keep my questions narrow.  I wish I could promise that I won't update on anything if you refuse but, realistically, I can't actually promise that."

Imperfect evidence is still evidence, and there are possible worlds where this test will yield useful results.  They may not be ready to defeat truthtelling spells right now.

"Okay, let's work under the assumption it's an honesty spell.  Does not prevent attempts at deception, does not enforce objective truth, you just can't say things you know to be false.  Um -"

Keltham isn't comfortable with this.

...people's lives and money are at stake, somewhere in the background, he really should do it anyways.

"I'm very sorry about even asking this, but I'm in very weird circumstances where I know very little about this world, so - are you alright with asking me one or two questions about this whole situation, as it is known to you, while you're still under a truth spell and there's been very little time for anybody to prepare for that?  I'll try to keep my questions narrow.  I wish I could promise that I won't update on anything if you refuse but, realistically, I can't actually promise that."

Imperfect evidence is still evidence, and there are possible worlds where the obvious test will yield useful results.  They may not be ready to defeat truthtelling spells right now.

"Okay, let's work under the assumption it's an honesty spell.  Does not prevent attempts at deception, does not enforce objective truth, you just can't say things you know to be false.  Um -"

Keltham isn't comfortable with this.

...people's lives and money are at stake, somewhere in the background, he really should do it anyways.

"I'm very sorry about even asking this, but I'm in very weird circumstances where I know very little about this world, so - are you alright with asking me one or two questions about this whole situation, as it is known to you, while you're still under a truth spell and there's been very little time for anybody to prepare for that?  I'll try to keep my questions narrow.  I wish I could promise that I won't update on anything if you refuse but, realistically, I can't actually promise that."

Imperfect evidence is still evidence, and there are possible worlds where the obvious test will yield useful results.  They may not be ready to defeat truthtelling spells right now.

"Okay, let's work under the assumption it's an honesty spell.  Does not prevent attempts at deception, does not enforce objective truth, you just can't say things you know to be false.  Um -"

Keltham isn't comfortable with this.

...people's lives and money are at stake, somewhere in the background, he really should do it anyways.

"I'm very sorry about even asking this, but I'm in very weird circumstances where I know very little about this world, so - are you alright with me asking one or two questions about this whole situation, as it is known to you, while you're still under a truth spell and there's been very little time for anybody to prepare for that?  I'll try to keep my questions narrow.  I wish I could promise that I won't update on anything if you refuse but, realistically, I can't actually promise that."