I guess not many people play any RPG straight by the book (and most, if not all, RPGs tell in the rules that they are mostly gidelines, I guess expecting most referees will house rule or at least adapt them somewhat).
What is being discussed is using an RPG rule set. Using the most of the rules and, most importantly the major set. House rules to tweak is NOT what is meant. The CORE rules of a given version IS what is meant. So, YES many people play with one rule set for many RPG's.