I'm currently encountering this predicament, but I almost always get around to finishing them. I still need to beat (just looking at my games list): Metro 2033, Farcry 2, FEAR, Dragon Age: Awakening, Just Cause 2, Penumbra: Black Plague, Spellforce 2, and Torchlight.
The only ones I don't care to beat are Farcry 2 and FEAR.
I still play Just Cause 2 and Dragon Age Awakening occasionally I just need to put in the hours, which I can't do thanks to work. A similar issue is arising with Metro 2033 and Penumbra: Black Plague, and it's eve harder for me to find time to play them, because it needs to be dark outside for me to get immersed into them.
Spellforce 2 and Torchlight will probably wait until the midsummer game release slump to be beaen, but I still intend to beat them. I just plan on beating my other games first, and I also plan on getting and beating Alan Wake, Alpha Protocol, maybe Splinter Cell Conviction, maybe God Of War 3, maybe Neir, and maybe other games I can't think of right now. Regardless, I will beat them all eventually, unless they truly are too dull to play, like FEAR and Farcry 2.