VoIP brings about the promise of a converged network, thus bringing down capital expenditures as well as operational expenditures. e.g. One network to buy equipment for, one network to staff for.
Those deploying VoIP dont treat the traffic the same, various Quality of Service techniques are used to ensure voice quality. Look up the stuff done on the IntServ camp as well as the DiffServ folks.
You can do both voice and data on the same links, you just have to know how to build your network.