I'll give my take here...
Can you tell the Average Joe audiences from the Rent An Audience shows?
Did/do audience "sweeteners" do a better job of audience reactions than paid audiences?
Is it necessary to show an audience on shows where it's not a requirement, or to prove there *is* an audience?
I frequently can tell the difference, and the audience sweeteners seem to me to do a much better job. As Tim said, the shows that are paying audience members are usually cable shows; in many cases, the "audience" seems to me to be taking up much more space then the actual set, and doesn't seem to me to be enjoying themselves at all.
So no, I do not think it is necessary to show an audience on shows where it's not a requirement. For some time now, my position has been that if you are going to pay a bunch of people to stare blankly at your itty-bitty set, you would be better off spending that money on some really good music (as opposed to the barely noticeable collection of beeps that passes for theme music on a lot of cable shows).
I myself have been in the audience for some thirteen television or radio shows that I would consider to be game shows, ranging from TPIR and WOF to a lot of public radio shows to being a volunteer seat-filler at my local high school quiz bowl show. I've never been paid for it; indeed, with the public radio shows, I paid for the tickets. Most seemed to me to be Genuine Average Joe Audiences, and not just in LA; until you go to a taping, you have no idea how loud the audience for Wait Wait Don't Tell Me is.