Even the same TV critics who hail this new form admit in their reviews that the shows are quiet, amusing in spots, the tone is more naturalistic, the actors are likeable, but there are very few big laughs. And that’s understandable because unless your cast includes Robin Williams, Sasha Cohen, Elaine May, Will Ferrell, or the Christopher Guest road company you are putting too much comedy burden on the actors. It’s not their job. It’s not their gift.
I think that's a terrific observation. There are a lot of great things about improvisation, but very few of them have to do with getting the big laughs. Think back to the comedy troupes that helped establish the importance of improv in comedy, like Second City. The point of improv in the Second City stuff was not, primarily, to make the sketches funnier; it's not like conventional sketch comedy was so short on big laughs or funny jokes that the younger performers thought they could do better by making stuff up on the spot. What improv lent to sketch comedy was a sense of depth and characterization; by acting more naturally, by forcing performers to really watch each other (because neither knows exactly what the other is going to do next), by incorporating awkward pauses and stammering and backtracking just like in real life, characters in sketches became more like real people than joke machines.
That's largely the point of the improvisation on a show like Curb Your Enthusiasm. That is actually a heavily scripted show; Larry David writes out what's going to happen in every scene, all the plot points, the subject of each conversation, all the big gags. The only thing that he doesn't write out are the actual words that the characters say, but those aren't the source of the biggest laughs, at least not directly. The improvisation helps give the show a naturalistic feel and it's also necessary to the theme of the show, because the show is about awkwardness and embarrassment. You can't write out and rehearse awkwardness and embarrassment; that, as Irving Berlin said, comes nat'rally. But when it comes to comedy, the actual choice of words is almost the least important part; the success of a show with improvised dialogue is perhaps a rebuke to joke writers who spend hours crafting the sound of each line (add more "K" sounds, quick!"), but it doesn't change the fact that good comedy needs a good writer. It just means that good comedy writing is about more than just funny dialogue.
If you go back even farther than that, some of my favorite movie directors of the Good Old Days (which I still call GOD for short) used improvisation that way: they'd plan out the scene and they'd write out the big jokes, but let the actors improvise the peripheral things. Leo McCarey did it that way; he didn't like to use a finished written script, but that didn't mean he relied on the actors to come up with the jokes. The biggest laughs in a movie like The Awful Truth come from things that were clearly planned and rehearsed in advance, like Cary Grant leaning back and falling off his chair. What the improv does is to give the performances a sense of naturalism that more tightly-scripted movies don't have, but it's still the responsibility of the writer and director to come up with the actual jokes.
Same with Gregory La Cava, whose film Stage Door is heavily improvised but still filled with great scripted punchlines ("Could you possibly see an older woman in the part?"). Improvisation is great in these movies, but they're not funny because of the improvisation; the best jokes are scripted, and the improv provides the depth and context for those jokes.
This post has gotten a little rambling and repetitive, but my point is this: I love improvisation in TV and film comedy. But what's good about it has very little to do with what makes those TV shows and movies funny. The funniest stuff tends to be planned out, which is why you need writers like Larry David or directors like Leo McCarey. But a good comedy needs more than laughs; it needs a sense that these are actual people, with lives and feelings and responsiveness to one another. And that's the element that improv can add.