From an experianced bettor:

The reason that Vegas will never go to moneyline betting (or straight up to win betting) only, is not just because it is confusing, and it is a lot stronger than simply "they want 50/50 action".

The real reason, is that if there were only moneylines, you would see games like:

Suns -400

Hawks +350

Essentially, that means a suns bet to win $100 is risking $400, a hawks bet of $100 wins $350.

The reason the books will never go to this system, is that heavy bettors will stop betting on favorites altogether. If you can make 3 $100 bets on large underdogs, even if only one pays off, your still up $50. If 2 of 3 pay off, you would see a significant profit on a very small investment.

Vegas already takes a bath on moneylines when a heavy underdog wins (the 49ers hit +450 ML's twice last season), but if they were to go to SU (straight up) betting only, they would quickly go out of business.