Remember the days when you played RPGs around the table in your parents' basement. There was one core rulebook getting passed around (or if your group was really lucky, one book for the GM and one book for the players to reference during play).
RuneQuest, Paranoia, Gamma World, Top Secret - the list goes on. These are some of the games I ran with just a single copy of the rules at the table.
Now, think about online play, whether via a traditional VTT like Fantasy Grounds of MapTools, or one of the Google+ Hangout Apps like TableTop Forge or Roll20. For a game to run smoothly, pretty much every player in the group was to have access to the rules. It's impossible to pass one copy among the players.
In effect, online play should be good for the publishers, at least those with core rulebooks for sale, as I suspect online players own a greater percentage of the rulebooks of games they play in than those that play face to face.
No, I haven't done a survey. I may at a later time. It's just that the situation and the facts make this the likely outcome.
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