Indeed. And perhaps it would be worth adding into the game. The only thing I'm thinking against it is this: We should probably try to minimize how many pings league has to keep communication simple and the controls for said communication accordingly simple. Now, the pings we currently have aren't always appropriate, but when they are appropriate, they are always useful.
You don't always need assistance, but when you do, you can always use the assistance ping. You don't always need to tell people where you're headed, but when you do, the destination ping always fits the need. Similar for the missing and danger pings.
On the other hand, the 3, 2, 1 ping has a weakness. When you want to coordinate the moment everyone jumps into action... It won't always work. It's great for coordinating anything with an arbitrary start time, or a start time of exactly three seconds, but anything that can't be declared concretely three seconds beforehand will leave the team either at a loss for a solid initiation, or vulnerable to the situation changing in an unfavorable way between 3 and 1.
I'm sure anyone who's ever been silenced for 2 seconds, or tried to land a skillshot with a long delay understands just how long 3 seconds can be in a match, and just how drastically a combat situation could change in that time.
I'm not against a coordinating ping to get everyone on the same timing as each other. Holy crap, I'd love it so much! But I'm concerned this version may not be as good as we want it to be.
I think we need something less rigid than a set 3 second timer, but still predictable enough for the whole team to be on board with what's happening. Something that works every time you need to synchronize.