Every project I do will have at least 5-10 concepts produced... If not more.
The client hardly ever gets presented with them all. My job as the designer is to curate the most appropriate solutions and show the ones that best solve their design problem.
The result of this is that over time you build up a kind of scrap yard of ideas that never see the light of day.
Every designer has experienced the client picking 'the wrong idea'... I just think, 'oh well, this could be used/tweaked/developed at some point in the future.
I've got files with dozens of half developed or rejected ideas... These are all assets for future use.
Don't think 'it didn't get chosen'... Think 'it didn't get chosen YET'.
Now you don't want to shoe-horn this into a project where it doesn't fit... That's lazy.
But if the right brief comes along, why not put it forward?
It's like a butcher watching to use every part of the animal.
Be efficient in how you work... Save every version that you do.
Nothing is every wasted and as Aaron Draplin says, vectors are free!
I love having an archive of 'nearly there' ideas.
There's nearly always something there that can be developed.
There's a lot of money in scrap, so make sure you don't throw anything away that can be recycled.