I'm about 50/50 on this.
I like the idea that it could make creating Timebomber levels easier, but I wouldn't be in favour of such levels being so precise that the exact distance has to be measured in this way. With that said, the proposed Radiation/Slowfreeze objects do perhaps promote the need for this feature, so it probably will happen.
A countdown ruler would need to account for many different types of movement: Climbing, Sliding, Falling, Floating, Gliding, plus Walking horizontally and diagonally (Swimmers, I think, move at the same speed as Walkers).
So, if this feature is to go ahead, what do we think would be the best design for it?