Reactive shooting relies on the photographer's ability to recognise an unfolding opportunity (observation) and to take advantage of that opportunity before the moment has passed (reaction). These are the hallmarks of street photography, because that's what's necessary in order to capture its images.
Successful outcomes of reactive photography are highly prized; and their virtuoso exponents, of whom Henri Cartier-Bresson is regarded as one of the foremost, are highly respected due, in no small part, to the recognition of how little control the artists have over their shooting environments.
This does not make reactive photography any more legitimate than its opposing counterpart, planned shooting. More realistically, reactive photography occupies one end of the shooting spectrum with planned photography at the other extreme. Between these is the 'half-planned' shoot, where favourable conditions are established, and then reactive shooting is allowed to take its course.