If you don't know the photo exists because you can't remember it, why are you looking for it?
Yes, I personally and professionally believe that proper captions and keywords make my photos recoverable, even though I organize them by date shot. I prove this on a weekly basis when somebody asks me for a specific photo, either with a general description, or by sending me a web thumbnail from which all the metadata has been stripped. I can lay my hands on the file within a few minutes. But then I am totally OCD about adding captions and keywords to everything that I shoot at work.
(And btw very little of this is within Lightroom itself. I started cataloguing with LR about two years ago, but my digital files go back to ~2000. I can use our online DAM product, or I can use Spotlight to search my server with all the raw files.)
At home I do have all my personal digital photos catalogued in Lightroom. I'm not quite as obsessive about captions and keywords, but of course I have many many fewer files. I also make extensive use of collections and especially Smart Collections, and I do spend time going through older parts of the catalog to re-edit and re-apply metadata. Even there, I can find a photo within a few minutes.