The thing about BOGOF is that it does not apply equally in every market. Yes, you're getting two for the 'price' of one, though of course you're accepting that the price you're paying is really for two of something, and that will have been factored in already in terms of pricing.
So let me turn it around. If Apple, for the sake of argument, offered you two iPhones for the price of one, would you take it? (Of course, if you don't like Apple, substitute your preferred phone supplier and preferred expensive phone model) Could you make use of two phones? Seems unlikely, but there are of course cases where buying in bulk does make sense, e.g. corporate buying. But as a general rule of sale, BOGOF does not work there.
Does it work for forum software? Good question. The real question, then: do the majority of users want or need multiple licences? Clearly some people do. I suspect, however, the majority actually do not, and so the real BOGOF model doesn't work (since then they're effectively paying over the odds for something they don't actually want or need)
The thing is, you're arguing about BOGOF but buy-5-get-5-free is not BOGOF by definition. 2-for-1 is quite different to 10-for-5, especially when you're dealing with a product that by definition is artificially inflated scarcity (it costs virtually nothing to create a new licensed of XenForo, per se)
The thing I can't help but think when I see your post is not 'this would be good for XenForo' but 'it would be good for me'. Here's the thing: Ashley was the business guy for Jelsoft, and clearly has an understanding of the market conditions in which XenForo operates. He would have known the figures for vBulletin's operations back in the day and it's not surprising that a very significant number of vBulletin licencees became XenForo licencees. That's a useful amount of data to have already, as well as having an understanding of how typical users actually run sites, i.e. how many licences per licencee.
The other question: will it actually translate into more money for XF in the long term? Answer is probably not as positive as you'd think. Let's suppose that a 10-for-5 deal did exist. Someone dropping enough cash for 5 licences is probably going to be busy enough running those 5 forums for a while and not expanding to 10 forums for a bit, which means it's possible that the first year's maintenance could lapse on some or all of those "free" licences. Which means of the 10 licences acquired, it seems unlikely all 10 will generate the full set of licence renewal fees. In fact, I'd almost go as far as to suggest they won't in almost any case: unless ALL of the original 5 purchases achieve some definition of success for the owner, they'll shut down the less successful ones and repurpose the licence(s), meaning that while there are 10 licences active, it is quite possible that only the 5 actually purchased will be in use at any one time, to the point where the purchaser doesn't gain anything, but if they do subsequently ramp up in future, it's going to take a while for XenForo to see any extra income out of those extra sales, if they do at all - as opposed to the current options for purchasing 5 forums.
See, part of the reason XF forums are generally quite successful is because of the investment. Gotta get your money's worth and all that. Free forum systems do not have the same buy-in and most fail at some point as a result. But if you're effectively encouraging more licences at no extra cost for the user, they're not going to have the same motivation, meaning that the extra forums may not be as successful anyway...
Long story short, I don't believe BOGOF (or even 10-for-5, which isn't really a BOGOF deal) is actually advantageous for XF, but I can see why it might be for you, even though there are already bulk discounts available.
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