I gotta be honest pablo... kinda harsh, dude! I have wondered the same thing about C lenses so I don't consider this to be a stupid question or a stupid thread.
So please, next time post something less condescending because your previous post does little to contribute to the discussion other than show everyone a side of you that isn't very considerate. I mean, really, did someone with a cine-lens kick your dog, call you names or something?
I stand by every word. They are specialist tools. I don't know how good or how bad, how rich or how poor the op asking the questions is. I paint a scenario of where these lenses come into their own, what they are designed for, what no other type of product would do. I think that goes some way to answering the question of why they cost so much... sorreeeee.
I also stand by my other comments, and nowhere do I consider them a personal attack on any other specific forum user, I am critiquing the fairly recent trend in photography where everybody wants the latest and greatest and most expensive. The technology has been static for 5 years as far as bodies go, and probably 10-15 years as lenses go. Part of that trend is confusing cost with performance or value. Simply put, a bright ring type USM lens will give a talentless photographer more keepers, but the photographs still won't be that great. Obviously they would give the talented professional more keepers too, but the talent would shine through regardless.
If the question was, will these cinema lenses make my still photography better, then, apologies if the timbre of my reply frightens the horses, but no, they will not. They will make your still photography worse.
There, I've just saved you 25k.
If anybody reading this has a spare 25k to drop on a lens, and will buy or not based on what somebody on a forum says... then I really consider that I'm doing them a public service. Find a charity close to your heart or something instead. It will be more rewarding.
Quite where you think I'm having a go a cinema lenses or cinema lens users, I fail to see. Cinema lenses are great. For cinema. And in that regard, the answer is in the question, so maybe 'obvious' is kinder than 'stupid'.
This is kind of fair and kind of unfair.
The CN-Es are by all accounts pretty similar to the Ls, not different enough for the 2% improvement in coatings or whatever and 9-bladed aperture to make up for the vastly huge increase in price.
The majority of what makes movie zooms movie zooms (parfocal, lack of breathing, mechanics) are helpful only for movies... but who wouldn't want an 18-85mm f1.8 (for APS-C only) that's as sharp as the sharpest Canon prime. (Okay it's 15lbs and $90k but still Fuji makes such a lens.)
I know of someone who converted a Zeiss Master Prime to his 5D or 7D for astro photography. If you have the money go for it, but as most just cover APS-C (other than the CN-Es, which, again, are not that much different from the Ls) it seems silly to spend up toward a smaller format rather than a bigger one. (IMAX uses modified Hasselblad and Pentax 6x7 lenses is the rumor.)
Also there is a longer history of still lens conversion than people realize. There was a 70-200mm Nikkor I think on one of the Bourne movies, Panavision has a host of converted Leica primes, some of which I've played with... they're nice! But the cost is in the rehousing... although the 280mm f2.8 Leica is sick.