My argument is that with the typical number of pulls people will have during the tier (which is likely to be way below 1000), you barely even notice the PPM difference caused by haste. You are right in that you don't know when a proc would have been missed with lower haste, but the same way you don't know when it wouldn't have been with more. If you look at the actual impact of haste on proc chance within the values we are talking about (usually max. 6000 haste rating), the difference is quite small. It would even out statistically if the number of pulls is large enough - but it usually isn't. That means that you very well might find yourself in a position of insignificantly more procs over someone with more haste, if you look at the total number of pulls throughout the tier. Plus, these procs are still subject to "bad timing", i.e. they may very well land in periods where you don't need them (like Lei Shen transition, or in mid-air on Ji-Kun, or whatever). Haste doesn't change the timing, in fact a higher chance only increases the likelihood of it happening (more procs total = more procs in bad spots, too). Mastery on the other hand retains its value throughout.
That being said, I'm only talking about haste's value for PPM here. Haste has other benefits, most of which are constant and not subject to RNG. I still believe that breakpoints are worth going for, if you have the gear for it. All I'm saying is that PPM is not a reason to go for haste; other things very well may be. But not PPM alone.