This seems to be the main arguement against a double check feature, however it’s a very personal issue. By that I mean, it only every affects at most a single user, and is entirely dependant on how they use the service. It has no greater bearing on the userbase as a whole. Someone who abuses a double check is only affecting their own progress and learning. Since there are no rewards for levelling fast or reaching 60, there’s no actual reason to keep such a feature from the general public because some people might abuse it.
That, and to say that people would abuse the feature, is simply pure conjecture to be honest.
There’s not really enough information around for us to make a proper measurement, but there are little pieces of data here and there we can use to make some guesses.
According to this thread the number of users is about 90K (active and past), measured mid last year. Since most apps require Wani Kani Open Framework, we can quickly look and see that that script has been downloaded 45K times, so we can assume that about half the WK users want extra functionality enough to modify heir user experience. (assuming unique downloads)
Using the same logic, we can see that Wanikani Double Check has been downloaded 21K times, so about ~23% of users make use of that feature. Again, reasonably significant.
I couldn’t find download stats for any of the major apps (most of which include double check and other new features), so I can’t really comment on them. This is particularly difficult since Tsurukame for example, is available from so many different places, it would be almost impossible to accurately determine how many people use it.