I've been asking this for many years and years because as John says, photographers should be able to define what files are part of their workflow.
There is not necessarily any need for LR to open/view/develop such files other than maybe a thumbnail. No different from handling a PSD. LR cannot open it but can happily pass it on to PS for content viewing/editing.
Not allowing a digital asset manager [which is what LR's Library is] to manage one's digital assets is a bit bonkers. It's an arbitrary decision made even more frustrating by the fact that even pro photographers are using many types of files such as video or graphics in their business. And having to mix and match file organisation programmes impedes the workflow that LR is meant to make smoother.
The grudging recent acceptance of video or PNGs only shows how daft the arbitrary limitations are, but suggest any change in forums and you'll be savaged by a pack of photographers saying "keep LR pure. Don't accept video or other files, LR is for photographers, burn the video shooters…
" and so on.
I find this outright hostility very strange. If you don't want non-image files in LR, then simply do not import them. It's not a tricky concept.
LR's half hearted video support looks even dafter now that you can process video in PS with ACR - which is the LR Develop module with a crappier layout.
Mind you add ACR/LR Dev to Premiere Pro and that'll make a lot of people very happy. Though I've also suggested a variation of LR [GradingRoom] that was aimed at video would be an amazing tool for complementing an editing programme like Premiere. Like Speedgrade, but good.