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It really depends on how often you will shoot in the rain, how hard it will be raining, how often you need to handle your controls and if you are willing to accept their are occasions where you may not be able to shoot (or wisely chose not to).
I too thought I was fine having an Optex sleeve in my bag until my most recent trip. Rained almost all the time. Not necessarily heavy all the time; drizzle, light rain, rain, heavy rain. Very high humidity. I REALLY wish I had something other than the Optex sleeve. the sleeve does not breath. It does not have an extra opening to keep your hand in, so if you need to adjust controls, a damp hand is going under the plastic. With the moisture on your hands and the humidity in the air, your camera can have moisture problems even if it doesn't get 'wet'. For me, it meant having the buffer transfer speeds drop to about 1/4 normal on my 7D at a very inopportune time. It also resulted in AF malfunction one day. Lastly, it resulted in three different occasions where the camera completely stopped working. I had to take the card and battery out and the lens off and let it dry for several hours before it returned to normal function.
The plastic cover approach (Optex or shopping bag) works reasonably well in some cases, but is risky in others and just doesn't cut it in some. A question to ask yourself is how expensive is your gear, how important is the shot and what is the risk (probability of occurence AND impact of occurrence).
Hope this helps.