It's
all variations of borders for the forumdisplay/threadbit table, not just removing border-right and border-left.
Some may want no borders for the tcats and theads, while borders for the tds or vice-versa (
especially if they want to take the challenge of replacing the tables with divs......READING THAT vBULLETIN DEVS??!!). All that above, shows the variations, and more importantly, where to find the tables to do so.
If vBulletin wouldn't hard-code class/id names, designers could transform those tables into divs faster, let alone remove the style="" statements out and replace them with cleaner class or id tags
at random.
It's still all ugly business. Non-practical, time consuming and just a plain waste of resources to use tables for presentation.