That is a very important item that many will overlook. To the person bemoaning what rights they will lose, well its not like your going to lose more, just be reminded of ones you lost and did not even realize, like that of a working cell phone.
What it does do is solve a political problem: "What the hell did you do in response to terror attacks" in a way that does't compromise liberty much.
It does make it just that bit harder to stow a bomb, because now the bomber needs to have it packaged properly, and hope that the package doesn't a) ruin the bomb or detonating device and b) that the bomb doesn't slip out and become visible.
It also helps the police identify - they can just machete all trash bags and grab/dump the contents and sort - compared with opening up the can - which might in some areas be locked or difficult to open.
Compared with moves like the creation of the TSA, I prefer this kind of security theatre - at least there's some plot here. It's like locking out the pilot doors from the cabin.
A real pain if you've got food and rubbish to dispose of.
The OP's point is that even if you can ensure 100% of the trashcans dont contain bombs for a reasonable cost, the terrorists will now just put them somewhere else.
France, in response to terrorist attacks in the 1995 mandated that all public locations have transparent bags held by posts [1] [2]. I don't think that's a unreasonable compromise, considering the alternatives.
[1] http://en.wikipedia.org/wiki/Waste_container#Media
[2] http://en.wikipedia.org/wiki/File:Trash_bin_in_Paris.jpg