Between a couple machines, desktops and a laptop, I usually have anywhere from 20-30 games installed, most of which I haven't played in months or even years.
Every now and then, when I get some time off, I try to go back and finish something. Games like Dying Light, for me at least, I finished the story and did all the available side quests, but didn't bother completing all the challenges, etc. I usually get sick of those semi-open world games after a certain point, and delete them eventually.
I was rather proud of myself a few days ago, in relation to this issue: I finally finished Warhammer 40k:Space Marine(cliff hanger ending!), Shadow Warrior 2 and Drakensang 2. Freed up a good 40gigs on one desktop, 10gigs or so on the laptop. Single player games like that, I can get 80-90% done and just let them stagnate for years. My characters get maxxed out, combat becomes ridiculously easy, then I struggle to finish it and/or something else comes out to divert my attention.
Of course 'new' games immediately popped out of the queue in their place, so not much SSD space was really gained, and the queue is still epic in length.
I still have major offenders like Star Wars:Battlefront on the SSD on this desktop, (55gigs?) and I think I'm sick of it already at about 10 hours played. I think about deleting that every time I turn my desktop on, but I keep meaning to get more involved with the multiplayer for it, but it never really happens.
In the end, it's no big deal to leave stuff on the drive or delete it, as we're all meaningless motes of dust in the universe anyway. If you may play it later, leave it on there until you NEED to make space for X new game that jumps the queue.