yeah i mean, its a legit argument to say that holy freeze provides defensive capabilities
the real question you asked is if its necessary to hit bosses. fundamentally, nothing is "necessary" given how OP warlock is. However, if your goal is to instantly delete everything, it may be annoying when ur in 8 person TZ baal, and his health isnt instantly being deleted bc ur heph is trying to hammer him without casting the amp curse. I personally ran a cursed holy freeze heph for many hours, and I did notice it a bit annoying it takes an average of 2-3 seconds for the curse to be casted (purely by him swinging, and then u get the 75% CTC and 45% CTH, it means that it may take liek 4-5 hits). However, with HF or HS, it is more or less instant. Again, not a huge deal sicne ur just deleting the map, but it is these cases where something isnt instantly deleted where u want to fix things. I now have a holy shock heph, and basically 8 person TZ baal dissapears instantly. I guess i have a higher chance of dying, but this is why i play the game
well another problem with bosses is that unlike normal monsters where you have shotgunning redundant curse aoe rolling a % for every target and putting an aoe around it, act bosses are still on their own, so you get that same 25% the holy fire/shock won't actually curse duriel/meph/diablo/baal (andy can be surrounded though, and ubers). Of course that's on top of him having the same chance with melee attacks.
but really we're talking about a difference of about 2 seconds on an act boss, versus a passive aoe chill aura that's going to be a boon in every other random area you're in, and could potentially save your merc from dying, save your demon from dying, save your char from dying. There's a few builds where people actively avoid holy freeze because they think it slows down kill speed- monsters running into hammers- but that's not the case for warlocks on any build really. Even blood boil has an aoe nearly as large as holy freeze aura anyway.
When the only calculation is "very slight upside of applying an unstackable buff to act bosses that you'll get in a few seconds either way" versus "slow everything in an aoe all the time" it doesn't seem very compelling