I've had a number of them, but like my Sitelite SL-500 the best. First and foremost, it has an actual switch to turn it on and off. I don't have to assemble it (putting in batteries) each time, then remember to disassemble it to take the batteries out so they're not dead the next time. I don't have to rotate a battery cage to activate it, and rely on the poor contacts to complete the circuit and actually activate the laser.
You won't be able to see the laser on paper from 100 yards away unless you do it in the dark.
Not true. My SL-500 uses a green laser, and I can easily see it at 100 yards, even in bright sunlight. Here at home, I usually boresight off of a neighbor's barn, that's right at 200 yards away. Again, no issues picking up the dot during the day (though, truth be told, his barn is usually in the shade).
None of them are accurate enough to save you a single round of ammo. You can zero any rifle just as fast, and with exactly the same number of rounds fired without one.
Not true. There are many rifles that can't be mechanically boresighted. The last two rifles I boresighted with my laser, an M82A1 and an M1A, both couldn't be mechanically boresighted. And, by laser boresighting them in my back yard first, I was on paper at 200 yards with my first shot.
It comes down to the quality of the boresighter. There's a lot of junk out there, true... and for those, I agree with you. But, a quality boresighter, used properly, will absolutely save you ammunition at the range. And, in the case of something like a .50 BMG, can save you quite a bit of money over time.