Usually you use left mouse button to select items and right mouse to give commands such as move and attack. The exception that caught me several times is when a building is being placed, which is using the left button. Since I consider the build action a command to the unit I tend to try to use the right mouse button for that. There are drawbacks with both. Currently I have to press ESC if I want to make a new selection. With the suggested change I must press ESC to abort building and give other instructions such as move.
As long as its a toggle it should be fine. Generally, most RTS use the left button for placing structures, so keeping it the default is a kind of requirement so people who are used to play different RTS, can play this without have to think about which button needs pressing.
I can't honestly think of an RTS that has right click for placing? Right click is usually for orders?
The way I see it right click is for two things; giving the default order and cancelling your active order. The default order is a context sensitive order and most of the time it is all you need. Left click is used for placing non-default orders and the most commonly used non-default order is structure placement. This is consistent with the other orders, if you click on the UI to select a command you will then have to left click on the map to give the command. For example the default order for right clicking on the ground is Move. If you want to build a structure, make a patrol route or attack ground you must click on the UI and left click on the ground. For non-default orders giving commands is the same as placing structures. This explaination is why I think the current system is more consistent than this suggestion.
Left click is almost always used for targeted abilities, such as nuke, reclaim requests, and structure placement. Right click is for context sensitive orders; such as right clicking on a husk to reclaim or on an enemy unit to attack. This is pretty common throughout the genre, and should definitely be the default scheme in PA. The OP's alternate scheme could make an interesting good UI mod though.
