I don't think a like or 'thankyou' button would be so bad. If you were to just say 'thanks' in a post it would likely be deleted, this would stop that occurring whilst also showing your gratitude to the person for the help.
It is nice. If there is a 'Thanks Meter' it would be very bad because they can make a few Alt-accounts and can click the button.
You have already said that in one of your posts:
It could contribute to your rep, but being helpful doesn't necessarily equate to they can be trusted to handle money risk free. If this was the case then people would start abusing it by creating alt accounts and liking their own posts and then using the 'look how many likes I've got - I'm very trusted' line in an attempt to scam.
IMO +1 and 'Thanks' are somewhat similar. Like you said, the button is still helpful:
If you were to just say 'thanks' in a post it would likely be deleted, this would stop that occurring whilst also showing your gratitude to the person for the help.
~~MZ~~