I agree with you that a lot of founders have catastrophic breakups in the first six months. In most cases, this kills the company, and good riddance. In the cases where it doesn't kill the company, the cliff minimizes years of painful conversations of the general flavor "Hello, 10th engineer. We are happy to offer you 0.2% of the company, which you'll earn over four years with us. This grant represents 1/250th of what someone who worked with us for two months once earned." or "Hello, potential acquirer. That certainly sounds like a very fair offer. Unfortunately, I'll need to run it by someone who I haven't spoken with in 5 years who you'll be writing a check for several tens of millions to."
Cliffs are, of course, negotiable. If someone suggested one to me with regards to my business, I'd accept it contingent on the clock starting at either launch or incorporation, either of which would result in the cliff being moot.