I've had no card failures, but was wondering if the feature appearing in most of the new cameras was because there is a real problem out there. It's also possible that Nikon perceives no issue but realizes that buyers will pay more for cameras with backup card capacity.
There is a telescope manufacturer (TEC) who used to put 3 light baffles into their telescopes (baffles are used to increase contrast by blocking stray light). After years of customers speculating that their competitors might have an edge optically because the competitors put 4 baffles into their scopes, TEC started putting a fourth baffle into their scopes. When asked about it, the TEC owner/scope designer said that they now put in three baffles to block stray light, and a fourth baffle for marketing purposes.