Sorry, a bit harsh. I think other people may have explained. Hobby isn't a common bird, and is always worth reporting. Somebody will be interested. Most birders only see flight views of Hobby anyway, and if they've flown over once they will often fly over again.
Even if there is no chance of it returning, it's still of interest to people, because it shows that they are (or have been recently) in the area, so it's worth keeping your eyes open, because it might still be close by.
Fly pasts of other birds such as Skuas are also worth recording, because they are usually fairly regular past the same points at the same time of year, and the passage often lasts for a few days. For example, if I see a report of a Pomarine Skua at Bowness in Cumbria in May, I know that the annual passage has started and though I won't see the exact individual, it's worth going tomorrow because there will almost certainly be more on the way. Same with Leaches Petrels on the Merseyside coast in September.
Sometimes birders don't just look for birds based on rarity value, they look for the spectacle of good numbers passing through, and return to the same places year after year.
It just seemed ironic to me that you weren't interested in a report of flyover Hobby, yet said you don't filter out common species in case you miss something you've not seen. It's almost like saying you're happy for common species to be reported when it suits you, but not when it doesn't.
