Way more valuable than a block. Not sure about steal.
Blocks are the most overrated. What percentage of blocks are rebounded by the defensive team? 100% of steals and offensive fouls directly lead to a possession for the defense. This analysis says only 57%: http://www.82games.com/comm16.htm
Apparently, 1 in 6 blocks go out of bounds.
I don't get why offensive fouls drawn are not tracked as a major stat, yet blocks are.
I guess that makes sense.
I guess the important thing with a block is that it's essentially the last line of defense. The best defense of all is one in which your opponent never gets a shot off. If no shot ever gets taken, then you can't block a shot.
That means that if you have blocked a shot, then the rest of your defense has been beaten, and you've allowed the opponent to get a shot off - and if that shot goes up, then there's a reasonably high probability that it goes in.
So regardless of whether the block ends up leading to the ball going out of bounds, or the ball going to the offensive team again, it doesn't matter - the important thing is that shot attempt got denied, and no points went on the board.
I know it's probably not as valuable as charges / steals because as you said, if you got one of those then the block would never be needed to begin with...but I guess the desperation of the situation that leads to a successful block is what gives it such perceived value.