Thanks! You are absolutely right. I am not speaking about SEO by the way.
In my opinion many people "browse" website directories when looking for websites. Especially now with website such as Alexa people always want to see "what is the biggest XYZ website in country XYZ"....
For example if you want to sell your car or buy a house, it is worth to use the top 3 websites in the relevant category, so that you see most ads.
If the category itself is incomplete, you are missing out on them.
I also understand DMOZ is a hobby by editors. If I collect stamps or cards my ambition is to have a "complete collection".
If my hobby is a list of the best golfing websites, my ambition is to have a complete list. Hobby is also about ambition. If there is no ambition, it is not a good hobby.
Of course some people are interested in SEO and pay thousands of dollars for a DMOZ entry or to get competitors websites removed. I have seen websites of companies who offer this as a service. One company offered the removal of a competitive website from a DMOZ category for 25,000 USD and the entry of an URL in certain categories for 10,000 USD.
In that case, DMOZ editors are certainly not doing this for a hobby.