I mean to me I read this as they want to find a way to bring in a greater number of people.
2 individual tickets to BGW/WCUSA = $85 at the absolute cheapest.
1 Fun pass for 3 days = $70.
Hit up the right families in the area that were 1 day-ing (if that) it to BGW get this card, maybe they enjoy themselves, become a full member because they tire of paying for everything else.
For members, this really isn't enticing. I got the Platinum for 2019. $347.88 is what I'll pay over the year. Doing some simple math based on what I've done in 2018:
15 visits to BGW, 5 to WCUSA, 2 to BGT, 2 HOS visits, 3 planned CT visits
So if I had the Fun Pass:
$70 for the pass
Parking Fee $15-20 per time, let's say I cheap it every time: $300 parking $370 running total
BGT Tickets are ~$80 each, $120 for the 2 day (IIRC) $490 running total
Let's say I manage to find HOS tickets discounted to $40 a pop $570 running total and another $30 for parking $630 running total
And my 3 CT visits, $25 per day based on what I paid for my mom last year: $705 running total.
And I don't get any discounts or perks that you get from having the membership. Not to mention potential blackout dates, not getting in on member only days, no extra passes for friends, no discounts for friends......to me the Fun Card makes sense if you are a family with young enough kids that they can't do a ton in FoF and don't want to ride too many big rides and you can go on non-blackout dates. Also makes sense if you live far away and are only getting in 2 or 3 trips in season.
But for a majority of people that have memberships to BGW, getting the Fun Card isn't an alternative that would entice me at that price.