No such thing as the most useless unit. All units have their uses, it depends on many factors such as situation, map type, current techs, your territory at hand, what you plan on doing, how you plan on attacking, who you plan on attacking, what units and defenses they have, how you want to defend, what leader you have, what type of terrain your fighting on, I can go on and on. This is Civ4, if you are a true strategist, you will know exactly what I mean. Now there are units that won't be built as much as others but none of them are useless.
A subtle distinction, but some people have indeed used the thread to say - I have not found any use for X unit.

And you don't get Astronomy from a hut every week or even month.
You're right of course not every hut will pop Astronomy but I would have been happy with any advanced tech. After I got the circumnavigation bonus my explorer was used to pop another couple of huts although they only gave gold they could have given a tech. The chance is worth taking just like it is at the begining of the game, huts can pop really useful techs, and you need to explore the map at that stage anyway, to find rich sites to colonise and barbarian cities to take (frigates, galleons and grenadiers versus axemen / longbow).