Some people are specialists, some are multi...um...ists. It's certainly possible, and I think employers seem to be looking for Jack-Of-All-Trades "designers" these days. But this kind of work does have a side to it that encourages specialisation. I, for example, can do the odd bit of graphic design, as well as basic video editing, but my main thing is illustration, and I tend not to market myself as anything other than an illustrator (no matter how many logo jobs I seem to do...) People only have so much time and energy to devote to improving themselves. Do you want to hire someone who dabbles across multiple disciplines or do you want number of specialists working together?