ok_aikido

Being 6' 4" has been nothing but a benefit for me in JJ. The leverage is awesome.

I think it depends on your reason for training in the first place. A child benefits from studying one art over a period of time, learning self discipline, etc. For self defense or combat situations, multiple arts can be a big plus. Over the last 20 years, I have studied multiple arts, some offensive, some more defensive in nature. In my opinion, being ready for any situation mandates well-rounded training. Training in multiple arts slows response time? Hardly. If you practice like you should, habits will form.