IMO, no speed means much unless we know the context. For club rides, I think average speed is useful, as has been stated previously to help riders decide which group might be best for their ability. But for touring, there are too many variables for any speed mentioned to be very useful, unless context is provided.
What I find useful about average, cruise, and what I think of as max flat ground speed is useful only to me when comparing how fast I seem to be going on different bikes. So, for example, when I transitioned from a Clark Kent race bike to my Slipstream I was able to compare and actually know that my average on both was similar, I was able to "cruise" on similar terrain at what I considered to be a sustainable speed, and I could tell that I could ride at 25 mph on both for about 4 miles, as long as I was on the drops on the DF.
What do these speeds mean to other riders? Perhaps not much. Or perhaps something. If a person reads that I ride those speeds, while he/she averages 20 mph or 6 mph, then he can gauge some things about what I say in a journal.
So if I ride a mountainous ride and say its hard and a slower rider reads that, he can decide how long it might take him or whether to go at all. Or a faster, stronger rider might say, "if Bruce can do that, then it'll be cake for me".