This is an aside, but this isn't the primary motivation for FP+. Or, at least, it shouldn't be for anyone who actually knows the numbers.
True, this was partly the motivation for developing FP in the first place. But, if you go back and look at the annual reports for the first few years of FP operation, you'll find that there was no unusual increase in per-capita in-park spending vs. the "pure play" theme park operators---at the time, Cedar Fair and Six Flags. People didn't suddenly buy more at Disney parks. For the most part, they just got in some other line.
Indeed, over the history of virtual queueing, Disney is the only operator who still offers an "included with admission" product. The other operators that had free VQ (Universal, Cedar Fair) have switched to pay-to-play, in some form or another, and pay-to-play systems have been adopted by/expanded at the other major players: Busch/Sea World, Six Flags, and Herschend. If VQ were such a revenue generator, you'd think the other parks would have tried to expand it without charging for it, wouldn't you? But, they haven't. So, perhaps it's not.
It's interesting to ask why Disney continues to offer an included-with-admission version. My personal guess is that this comes down to Disney placing a higher value on good will in the balance sheet. Over time, I do expect Disney to start monetizing FP+ in some more direct ways, but so far the public statements by the company suggest that it's going to continue to be a level playing field---most notably, in the interview Nick Franklin gave to an Orlando TV station a while back.
