Let's say you have a covered call on AAPL. Your short call is a Dec 400.
As Dec 17 gets closer, the price of that short call will fall... eventually getting to zero when it expires. Let's say it's Wednesday Dec 14, and the option is worth $.15. The most you can make by holding that option is $15 more. If AAPL for some reason gaps up $20 the next day, you could make $2000. So the benefit you would get ($15) is hardly worth the benefit you would lose ($2000) even though it is unlikely. So the risk/reward of holding an option to expiration isn't great.
Another way to look at it. The short call option provides downside protection. If I sell a call option on AAPL, for $10, I am gaining some downside protection. If AAPL falls $10, I'm at break even. However, if I wait till the call option has decayed to $.15, then I am protected to the downside by $.15. Big Deal. It's not worth having essentially no downside protection and limiting my upside. Again... close those expiring options several days early, perhaps a week early if they have lost most of their value.
John