as long as you download it and you are a plus member you can play it and as far as i understand if you stop plus and then get it again you can redownload it and play it again. but if you never bothered to download it before it leaves then once its gone you are out of luck.

in fact you dont have to fully download a title some games i think i might want to try out in the future but not use the hard drive space now. so just start the download and after it downloads like at least 1% cancel the download and if you view it it will say purchased and you can always grab it later.