Having used LIFs a good bit around military hardware, IMHO, the performance loss is noticeable enough that unless I was really going to be around the types of equipment that LIFs are supposed to provide protection from, I would avoid using them, and they can be especially problematic if used with many units that have visible green lasers like the MAWL, RAID-X, and many DBAL models.
The Wilcox thread-in protective cap uses a clear, replaceable lens and costs around $30 with no questions about source or legality, and less reduction in performance for the same amount, if not more actual physical barrier protection to the optical lens than an LIF.
The clear press-on sacrificial lens cover also serves the same purpose, and you can usually find those second-hand/surplus for a dime a dozen--most lens accessories that you might use that you can't use with the press-on cover come with sacrificial lenses themselves, like the Hoplite or Tarsier.
Honestly, considering that they're not going to protect you from most common lasers or radiation sources, they're often expensive and almost always questionably acquired, especially if cheap, I think that most people like LIFs either a) because they don't understand what they do and don't do, or b) because they look cool... when you're looking at the googles, but not through them.
::shrug::
~Augee