Since every vendor has a MAC address range, and Apple has network card models that only they use (Airport), I suppose that it is very easy to identify a network package as coming from an Apple device, even if the traffic is encrypted.
One of the most valuable things an attacker must find out, is which operative system is running the computer they want to hack.
Therefore my theory is: Changing the MAC address of an Apple device to look like the other of a non Apple vendor (and viceversa), can improve my security within local networks.
How foolish is my humble theory?
UPDATE: I know that this alone will not solve many security problems. I just wanted to know if being concerned about my MAC address is a waste of time or one more good point to consider when tuning the security of my devices.