Well as I said in the post you replied to, this change results in a wider range of gameplay that's optional. Which to me seems a clear benefit.
You can put it in terms that try to frame the motivation as something you think is bad but if it results in more interesting gameplay then who gives a monkey's.
I don;t see a direct path from increasing influence payouts in open for PP to more interesting game play. It's a wish, but not a surety. Suggest some wider range of game play you expect to see developed from an influence modifier for those that play in open. Do you expect a bunch of Solo/Group players to suddenly play in open for such a marginal bonus? I don;t. I argue against this notion more on principle than on it's results.
Once FD are seen to officially deem open worthy of some kind of bonus, the precedent and the demand will be there instantly. You can see this already in this thread. There is no reason to have to suggest this as a slippery slope issue, when players are already sliding down it in glee.