We've grown fond of the characters, which saves the film a bit - on its own, without the backstory, it'd be much less effective. If anything, it's at least a relatively enjoyable experience that you can palate for a short time. Think of it as going down easy in small doses, like eggnog.
The law of diminishing returns is a law for a reason. And nowhere are the returns more diminished than Little Fockers, the third film in a series that began in 2000. With this one, they've barely bothered with a plot.