Where any choice exists there is a preference, it may not be strong, it may not be exclusive and when it comes to cognitive functions its very unlikely to be netly divisable but there's still a preference, there's still something that's more comfortable and natural to a person than another. So like it or not it simply is. Hence there's no invalidation from that stand point.
To me the whole tendency to rubbish MBTI on the basis you mentioned speaks back to the whole consumerist cultural malaise, choice was great, now its too frustrating, why choose which involves foregoing alternatives, accepting a marginal cost, instead of rationalising that you really can and should have it all instead?
You could argue its insufficiency or subjectivity or conceptualisation is inexacting and those are the reason why typology or the whole of psychoanalysis is often dismissed as of purely literary value but it works for a lot of people and is at least a place to begin thinking about mind, motive and behaviour.