What a lovely shot, I am sure they will cherish it when they grow old.
As far as I understand, sharpness is not an ultimate goal but just one of the tools. All photos must be sharp enough. How enough is "enough"? It differs in each photo and depends in the genre and artistic idea. If you are a reporter, your photos are expected to be very sharp, street photography and even portraiture give us more space to imagination and artistic freedom and hence more varied degree of unsharpness. But in each and every picture there must be a clear idea behind it.
Lev's portrait is just sharp enough, it is a romantic ( provocative even) interpretation of two beautiful girls at the age of transition, and the shot reflects this mood beautifully. One could take another approach and document, not this unique and important period of adolescence, but a particular day or a moment worth remembering, and then we would need an immediacy i.e. a sharp picture.
I must add though that with street photography where documenting a moment (as with HCB) is a prevalent goal, unsharp images 99% of the time are just photographers' mistakes or a result of exposure limitation due to low light or fast moving scene, that are sometimes excused afterwards by "artistic vision".
At least I do not remember a single street shot that I have taken deliberately trying to make it unsharp to a certain degree, this is beyond my level.