It might sometimes be nice to know these things, but knowing them certainly is not "amazingly invaluable". Precise damage numbers are omitted because it would be meaningless information spam. The number of exclamation marks at the end of the hit message specifies a certain damage range (this is not obvious at all, and also generally not very useful). Knowing the stats of a monster (such as base damage, exact speed, and whether it has high AC or EV) is very useful. Knowing how much damage a monster just did is not. Damage first goes through a to hit roll, then a highly random damage roll, and then is reduced by an AC roll, whose minimum is modified by guaranteed damage reduction for melee attacks on heavy armor characters. The end result will give you very little information about the monster. What matters is how much damage a monster *can* do, not how much it just did. If something hits you for less than 5 damage 3 times in a row it might still hit you for 40 damage the next turn.
A lot of monster descriptions got a lot more descriptive recently, and AFAIK one of the goals of the text improvement taskforce was to give unspoiled players more information about what each monster can do. I don't know if the new monster descriptions are in 0.10, you could try playing trunk (unless you're playing webtiles
).
I'm sure buff durations are deliberately hidden until the buff is about to end so you don't know when it will end (unlike hit damage, which you only get a rough estimate of because nobody should care). Note that buff durations are very random. Some spells have a much longer average duration than others, but the duration roll is usually something like 10+1d(50), so only a very small fraction of the maximum duration is guaranteed (also the 50 generally depends on spellpower).
Spell failure did not give exact percentages until recently. It was changed because the words used to describe spell success were amazingly confusing, and because the difference between 2% and 4% failure is actually relevant for many spells (but irrelevant for many others). The coloring for potential miscast severity was also added, because it is relevant information.