I was talking to a person who literally had over 1000 different domain names under his portfolio. Basically, he registered all these names back in the day when domain names were selling for a whole lot of money. Since then most of them are now just regular domain names where no one is likely to pay say six figures for it. However, he continues to renew all of them just incase something does come up.
In many ways it kind of reminds me of various types of insurance where you continually renew it as it is one of those just incase scenarios even if it is something that is rarely likely to happen. Makes you wonder where you draw the line though. Like in this case I think you are more likely to make more money simply by not renewing everything all the time since the chances of selling it at a high price is so low.