I was just about to post the same question. There are all kinds of actions now that can generate a "one-click thank you". This is part of the glue that makes WikiTree a community.
Being added to a profile's trusted list is a big deal, and when a profile manager agrees to add me, I consider it a big deal that is worth a thank you. Yes, I can write an individual note, but I thin a "one click thank you" would be very appropriate and in keeping with our WikiTree culture.