Here's the thing with adding cameras to the scene: You effectively multiply the geometry in the scene for each camera, and you need to cull the parts that are outside the camera bounds, each of these operations costing processing power.

The core element of a digital 3d scene is a collection of vectors that define points and their x, y, and z coordinates in three-dimensional space. Every object consists of vertices (points) described by those vectors, edges that connect points, and surfaces (polygons) that are spanned between these edges. These polygons are then (if necessary) divded into triangles, and those triangles then get rendered.

If you look at a simple cube you can imagine how that works: Each corner is one vertex (8 vertices total), you have 12 edges, and 6 quadratic faces. If you look to turn every face into a triangular shape you need to cut every face in two, so you have 12 triangles.

Now to render this, you have a camera in the scene. You calculate the coordinates of every triangle in relation to the position of the camera, and then you do a step in which you change every single vertex again in order to calculate the perspective (closer objects are larger, further away objects smaller). Also you need to cull (cut) the vertices that cross the edges of the camera view port so you don't render things that wouldn't fit on the screen.

You would have to multiply this last step for every single camera you have in the scene!

It worked for Portal (where you have your player's view and up to two portals), yes, but that is because it is a game-defining element of Portal! They were able to design the game objects in such a way that they would use very few polygons, and the levels so that they had a low object density. In fact, if you look at Portal 2 you should notice that those parts of the game where you could look very far and many objects were, were exactly those parts of the game where you could not use any portals!