Yes, some packets should have higher priority, but no ISP will be able to trust applications to set priorities correctly.
What an ISP could do is ensure that you get a certain amount of high priority bandwidth, and then leave it up to the user to decide how to use it. Likely, the user would need a router that sets VOIP packets at high priority and everything else at low priority (or something like that) because you probably can't trust your own computers to set priorities.
If you leave it up to the ISP to decide on what's high priority, few ISPs will be able to resist the temptation to slow down compe ors, e.g., slow down VOIP compe ors, slow down Google in favor of Yahoo, slow down video in favor of your ISP's cable channels, etc. Now the ISP can try to sell faster service to its customers, even though everyone is already buying access to the internet, and the ISPs already have agreements to transfer traffic between them.