Just would like to know.
I would feel really restricted if I had no zoom.
It depends on the game.
Company of Heroes is a game where I don’t feel I really need it, so don’t use it much; the camera height is fairly good. Supreme Commander is a game where I need to constantly jump backwards and forwards between different camera views as part of the core gameplay.
Command and Conquer Generals is a ****up where I need to modify the camera ranges in the ini files, because even the maximum camera height is far too low and you can barely actually see anything.
I personally like the zoom key as it will allow you to go near your characters (you can see more details) 
Will have to agree with the rest; it really depends on the game.
I think there are two reasons for zoom in RTS games:
1: ‘Aesthetics’
2: Strategy
The first one, as @ mentioned, because some people like to view the characters/buildings/landscape from close by or from far away - but solely for the way it looks. This is arguable not necessary, but could really make a game more enjoyable to play. I personally like this in games such as Age of Empires but don’t really care for it in C&C.
The second one, as @ambershee mentioned, because it is really necessary for the gameplay and strategy. This is very game-‘style’ specific, and not allowing zoom could completely change the game. For example zoom could be used as a tool to allow players to plan strategy, or as a tool to add some form of handicap to the player (in the case of very restrictive zooming).