I can't speak for llimllib, but I didn't read their comment as being a reward for having a bath. The key bit of the suggestion is that having a bath is implicit in the whole situation.

Similarly, me (+spouse) have never offered any reward for completing any task - and see many other parents having to enter into a what-if kind of negotiation. I like your Mercury example, but would consider taking it further: "I can't tell you about Mercury until you're dressed". It sounds logical enough to a 5-6 year old (even though these things are not logically connected).

I guessing these tactics will have to change at some point - but we've successfully navigated around most of the tantrum stage so far.