Whatever yeast is still in suspension will continue to ferment, but likely at a slower pace according to what I have read here. The remaining yeast may also not be up to the task of cleaning up after itself. Unless you feel like cleaning a second fermenter or are racking onto fruit, dry hops, secondary yeast, etc, using a secondary is not nearly as necessary as once thought. The few times I have secondaried, I did have attenuation problems and/or off tastes and flavors. Those were also before I had good temp control as well, so read on to hear what everyone else has to say. This is a pretty hot topic on here.