Why? What do they have to loose? It's not some kind of information you can't get from the SERP, so I don't see any reasons. Google has many options on the search page? Why not one for hiding/displaying the numbers?
When I want to see at what position my site appears on Google SERP, I choose to display 100 results and use the browser's search function to find my site (hopefully, on the first 100 results). When I find it, what do I do, count manually on what line is it? By the way, I found a tools that helps me with Google SERPs: http://serp-easy-surf.seoquake.com/index-en.php
If you prefer them numbered and use Firefox, there's a Greasemonkey script that does it called Number Google Results Remixed. Very handy when I want to know my rank at a glance.
Well, sometimes it was number 11 . Especially for new sites, it makes a difference for me if there is an increase from place 900 to place 300. I started this thread because I thought that if Google doesn't display it, there is a reason for it. Or, if there is an option, it might be buried somewhere and I couldn't find it. Thank you very much for suggestion.
I quite like to see the numbers. If I am searching for something and realise I am down to about #30 I might decide to search for something else. So I guess with no numbers, it might encourage people to wander further down the list ?