ADC ENOB? Throw away extra bits?
Just because they are noise doesn't mean that noise doesn't carry useful information. Have you ever heard of a statistical mean and variance? Even though the last few bits are noisy they are still noisy around the statistical mean and carry information about it.

Some ADC's still give you the last bits even though they are noisy for averaging purposes.  

Its really dependent on your application, ADC and the noise distribution you are working with, but you can also gain usefulness out of the noise and decimate and average an integer number of samples together. If you do this by a factor of two all you need is an adder and then you throw away the last bit. You do this at the cost sampling time\bandwidth.