Having a true toggle (like the ALT+F type toggle for colors) for HP bars would be extremely beneficial to players, analysts, spectators, and commentators. Everyone's favortie guy Day even mentioned a while back in a daily that it was very hard for him to "parse" all the hp bars. So I know I'm not alone in this.
Players have to use HP bars the majority of the time, but situations arise where you need to see the units without the clutter. Like in FF at ramp situations, late game mass army battles, ling vs. baneling, marine micro vs. baneling, etc. The current ALT toggle is only temporary, and is not practical when you have a split second to cast FF, avoid a baneling, etc.
When watching videos or the GSL, there are plenty of times when turning HP bars on for a while or off for a while make a lot of sense just for ease of viewing.
Why is this not a true toggle option? So I thought it was too late for this, but Blizz is still making major improvements to UI. I would love to see this addition!