I'm not talking about the game judging the value of a rune. I'm talking about stats that are completely useless. Toxic Damage on Megataur. Improved Shields on Moon Drake. Skill Power on Bard.
Yes, there is currently a way to figure this out for yourself (most of the time), but people don't even notice the 'i' button, let alone want to sit there reading help screens. Just look at this forum. Half the non-complaining posts are people asking for information they could easily get from the game. Why not be nice to those players?
Why not be nice to the likely large number of players who are under 10 years old, and don't have a strong sense of strategy, or the willingness to pick apart stat screens and help screens?
I've made some fairly difficult-to-implement suggestions in this thread, but this isn't one of them. A table of which stats are useless for which heroes is easy. Crossing out stats on runes that a hero can't use is also easy. It would be easy to notice when a player was adding a useless rune set as well. Bards shouldn't have Ocean.
Rating rune power would be nice, but as you say, very difficult to implement. It's easy to figure out rock, mist and tree, but is tree+rock better than ocean? That's harder to say, and could result in suboptimal suggestions based on developer opinions.