Generally, we need a bunch of ADC channels and (usually) only 2 DAC channels for monitoring right. So many converters or all-in-one interfaces have equal amount of ADC as DAC like 8 of each. Why would you need more than 2 DAC channels other than surround mixing which would of course need that many channels. Are there any other uses for that many DAC channels? I am thinking I could get away with getting a separate ADC rack, and a small 2 channel DAC box, and save some money, but I dont want to learn later that there is a good reason for having so many DAC channels.
Also where do headphone outputs fit into all this? is every headphone out another DAC?
You can use them for separate headphone mixes, sending audio to outboard processing gear, send individual track or groups to a mixer. There are lots of different uses for DACs than just your monitors. And yes, the headphone output (just like on your iPod) has a DAC as well.
If you want to leave your options open, I recommend getting an all digital interface such as the RME Raydat, AES or Madi Card. Then you can upgrade and downgrade your converters separately as you need. depending on the application, you don't always need all your converters to be expensive either. for instance you could pick up a Behringer Ada8000 and just use the DA converters for your headphone outs, but have high quality converters on the inputs and 2 channel out.