I came up with the lamp rod eyelet mod back when I got my 22.5" WSM years ago - I was to cheap to buy the Cajun Bandit ones... lol. They have never made any impact (that I can tell) on my temps, and they have never caused any rust from drilling the holes.
I did see one thread where somebody also purchased the lamp rod caps and used them to cover the hole when no probe was being used. Or as Hambone pointed out a lot of folks also like the "slot" method as well.
Glad to hear Weber is making them a standar offering now on the new WSM's.
(the link bellow my signature is to my original thread for doing the lamp rod mod. if you want to look at it)