I think this is an interesting addition and likely will fill a gap in many beginning Japanese learner’s study routines!
However, I also feel I should mention, as I have seen others in similar situations on the forums before and I don’t know that this was sufficiently covered by previous replies in this thread (though many mentioned a similar issue just as advanced users of WaniKani): there are users who come to WaniKani after years of studying Japanese from other sources because it is resource to specifically learn kanji.
I am one of those people. Personally, I took Japanese classes at the high school and college level, did two summer terms studying abroad in Japan for 1-2 months, and even taught English in Japan for a year while continuing to self-study Japanese. I was still horrible at reading kanji.
I started WaniKani because I had always had problems getting kanji to actually stick in my long-term memory (often learning them for a quiz or test or paper and then forgetting them after). Most other kanji-learning resources I found introduced kanji strictly by frequency or grouped by a common “theme” and in isolation, with the idea that just writing the kanji a bunch of times will help you remember it.
WaniKani’s method of both building up kanji from component parts while balancing frequency of use (other resources such as kanjidamage, kanjigarden, or “Remembering the Kanji” build up much more strictly) but more importantly for me drilling in the meaning and pronunciation with related vocabulary words. Often, because of my prior knowledge of Japanese, these were also words I already knew and so really helped to cement the kanji.
Yes, I had some frustrations in the first couple levels with kanji I already knew and no option to skip or mark “I know this” but WaniKani quickly proved after that to be a method that worked very well for me to finally really learn some of the kanji that I had never been able to commit to long-term memory in all my previous years of wrote memorization.
Even with that forcibly slow beginning stage, I was still happy to recommend WaniKani not only to Japanese beginners who had just finished learning hiragana and katakana but also to people like myself who had been studying the language for a long time but still struggled with kanji in particular as I truly believe it is one of the best resources out there for that (other sites/book mentioned above or White Rabbit Press kanji flashcards also being great options).
I love what WaniKani is trying to do here, but I think this addition (again, assuming 100s or 1000s of such vocabulary is added) will make me a lot less likely to recommend the site to people in a situation similar to mine, because it will just make it a lot more items to slog through to learn what they actually want to know. An option to “mark as known” or just skip the kana-only items altogether would be very nice so I could continue to recommend the site to friends with prior experience learning Japanese who just need help with kanji.
Personally, I would love to see an “I already know this” button option for ALL items (@MissDagger’s suggestion that such a button not automatically set the item as “burned” but instead set it to Master or Enlightened would be interesting to test out) to make the platform more appealing to Japanese learners who come in with a lot of prior knowledge. Many other sites do offer something like that. I am currently using the [Userscript] Item Filter to somewhat achieve this functionality, but it does have limitations.
As a side note, if WaniKani is aiming to be even more beginner-friendly, maybe adding some “pre-kanji” levels for learning hiragana (with just pronunciation cards + some kana-only vocab, maybe based on the Tofugu “Learn Hiragana Book”) would work well and that could be separate from the kanji levels.
(minor grammatical edit)