Eta: America's system works quite differently than Australia's. Being 'registered' to a party doesn't really take up my time, it's a box you check when you first register to vote, just means I receive the democrat primary card and that when I vote it shows up that a 'democrat' voted for such and such person or measure.
Eta2: I just read the manipulation area where the worry is more that another party organizes voters to vote for the other party to 'dilute' it or chooses somebody more easy to defeat. On the other hand it gives people who do not affiliate with a particular party a chance to have their say. Again though, it's 16 states out of 50. I'd prefer them all to be closed but overall I like how our system works, that we get to choose our candidates.