I understand what you mean. And the collision example is a good one to demonstrate it.
When a collision occurs, there are 2 objects involved in that situation. For ex. a player and an enemy. If we focus our events on the player and create the "Player on collision with enemy" then we could reference the other object with this expression (in this example, the enemy). Yes, it can be done with families, but not in the free edition. Also, you don't have to create families for different groups of objects if you can use the "other" expression. It may help developers who think differently to those who use families efficiently. Also, the collision is just one example, I'm sure it would be useful in differens situations as well.
What if there are 5 other conditions in the event which all refer to different object types? What if there are hundreds of "other" instances to choose from? How do you know which will be referred to by "other"? This seems far too ambiguous to be useful at all.
These are indeed problems with this idea. Maybe it should be a Sprite only expression. I don't think it would be a problem if there are hundreds of instances. Staying with the example: the collision event would run for every collision, so picking the other instance there can work.