Everyone's comments are right. If you want to eek out every possible ounce of performance, arrays are faster, but for 99.99% of the jobs, hashes will do more than what you need, and they have other benefits in terms of object inheritance.

What I'm surprised no one has touched upon is:
Part of it is laziness: typing "[ ]" as opposed to "{ }", which requires a shift key.

Can you explain to me exactly how you're typing '[BAR]' without using the shift key? I suppose you could hit the caps lock key, but again, no decrease in the amount of work done.

As an aside, I did implement a class using pseudo-hashes (still use it in fact, very robust). While the whole idea may be very bad, it's worked out well for us. Our performance bottlenecks were elsewhere, though.