I am not sure that a view with alpha 0.0 is still drawn. Check the documentation library:
Hiding Views
To hide a view visually, you can either set its hidden property to YES or change its alpha property to 0.0. A hidden view does not receive touch events from the system. However, hidden views do participate in autoresizing and other layout operations associated with the view hierarchy. Thus, hiding a view is often a convenient alternative to removing views from your view hierarchy, especially if you plan to show the views again at some point soon.
I also have found this answer here http://www.iphonedevsdk.com/forum/iphone-sdk-development/65525-whats-difference-between-alpha-0-hidden-yes.html
That says:
I believe that Cocoa Touch treats and alpha less than 0.02 as also being hidden, since below that alpha level it’s invisible, and Apple’s engineers decided that invisible controls should not be clickable.
Using an alpha value requires that the graphics hardware blend each pixel from the object with everything underneath. It’s compute-intensive. The hidden flag, on the other hand, is a switch. If you turn it on, the OS knows it doesn’t have to draw the object at all.