My problem with the 15mm lenscap was: why would I use it instead of the excellent Panny 14mm? The 14mm is just slightly larger, way sharper, AF, and you can actually change the aperture. So after the novelty wore off I sold it and haven't looked back.
But the 9mm's a different animal. The only "replacement" is the Samyang, which is a great lens but MUCH larger. I can easily see times when I would take the 9mm along but not the Samyang, simply on the basis of size. This is why I'm intrigued by this lens.
You know, you are right in a logical sense. I bought the 15mm BCL to use as BODY CAP, but I've found I'm treating it more as a lens. In my camera bag, I have a few little pockets for filters where I always keep a spare body and rear lens cap. I bought a $35 USED 15mm BCL purposely so I can throw it in there and not worrying if it gets scratched. The $200 14mm, although it will fit, I don't want scratched. Plus, I have this thing where if I have more than one lens on my person, I'm not a happy photographer because I keep wanting to change lenses. So, the BCL, similar to a plain body cap, with its lack of sharpness, prevents me from using it. However, it's always there for those times when I want to remove all lenses and still have something to protect the camera. It sounds weird but I wish the guy who sold me the BCL would have been a bit more rough with it!
The 9mm BCL, however, I think would be more useful and will also fit where the filters go. Yet I don't think it will have the bold presence (read: quality) of the 14 or 12-32, meaning mentally, I won't feel a desire to keep pulling it out. But at times, when I'm out with a 50mm and get bored of that a small fisheye that weighs nothing would be nice. Perhaps in a few months someone will sell a beat up dirty one for under $50
Unfortunately for me, people's emotion is why I think nearly ALL lenses are useful, no matter what lenses you also have. I own the following:
* 14/2.5 - you'd think this would be useless if you have the 15/1.7 or 12-35/2.8, but the 14 is still smaller and cheaper
* 11 UWA converter - I thought that after buying the 12-32 this would be useless, but it's quite a bit wider when used on the 14 (comparisons to come), about a half stop faster, and is safer to twist on the 14 than to remove the 14 and put the 12-32 on.
* Oly 14-42 - after having the 12-32, this lens seems useless. But, it does have extra reach. Another lens I SHOULD sell but haven't.
* Sigma 30/2.8 - I have a Vivitar 28/2.0 which is one of my favorite lenses, but there's something I like about the Sigma. AF, smaller size, overall sharpness, and plastic touch in the cold, perhaps? I should sell it. Even if you had the 12-35, the Sigma is smaller; and I'd like to see some samples because I can't imagine the 12-35 being better, not at f2.8, at least.
Some lenses I do think I would never use, but do have their use
* Sigma 19/2.8 - I guess price is the only reason you'd take this over the 20/1.7
* 17/2.8 - I guess it's a touch smaller than the 17/1.8 and wider than the 20/1.7, but I'd never get it. It'd be a good starter lens for a kid or GF.
* Kit 14-42s - With the Panasonic 14-45 and the 14-42 version II being better, I'd say these are complete replacements for the kit lenses. Fast zooms too, if you had them. But then again there are places I wouldn't take a $1000 lens where I wouldn't mind taking the kit. plus, the kits are free
* Pan 42/1.2 - I know a lot of people would take this over the cheaper MF Voigtlander 42/0.95, but I'm not one of them. Ditto the 42/2.8 macro.
* Oly 25/1.8 - Not if you have a Panasonic 25/1.4. But if you don't, I'm sure the price will be about half; plus the lens is smaller and is more contoured and looks as nice as the 45/1.8.