I heard the reason XC don't have catering is due to the staff being based in Edinburgh, ie an operational reason. As a passenger it seems strange that the long distance train from Aberdeen doesn't have catering yet local Scotrail trains all do. People expect catering to be provided these days, especially on the long distance trains. As for demand, the XC trains are often very busy. I've often seen them standing room only leaving Glasgow. There would certainly be a high demand for catering. People travelling to Edinburgh from Glasgow often use the XC service due to the ability to reserve seats and being slightly cheaper than Scotrail. Plus It only takes about 10 mins more.
Yes, people will probably have breakfast before they board the 0820 from Aberdeen, but an hour or two after leaving their house they may want something else to eat or drink. The VTEC services either side of this departure have hot food onboard. Scotrail currently have a trolley but will be getting buffet coaches with the HST's. In terms of catering, XC really do lag behind their competitors in Scotland.