As he says in the article, functions should be small but real world code isn't always written the way you'd want it to be. I think instant feedback would be great for understanding a piece of code that I didn't write but the chances are that if I don't understand it it is probably badly factored with large function sizes and mixed responsibilities all over the place.
Ultimately there will be code for which this feature is useful and code for which it is not. I can't say I wouldn't want it to be in my IDE's bag of tools though.
It should be annoying as possible to write/read long functions. I don't know why IDE's/editors make it so easy to write 500 line functions; it's always bad. I interview over 10 programmers/day and function size is a huge filter; you recognize crappy programmers fast that way.
Ultimately there will be code for which this feature is useful and code for which it is not. I can't say I wouldn't want it to be in my IDE's bag of tools though.