To Cross Train Or Not?

As I search through the thousands of writing jobs (freelance and staff positions), I see again and again that a huge percentage of the jobs require desktop publishing, web site design, or photography experience in addition to being able to produce quality content. The benefits and job opportunities for a good writer with skills in any/all of these areas seems endless.

But wouldn't studying all of these fields make me a "jack of all trades and master of none?"

I hardly have enough time now for writing. Should I push myself harder and train in these other areas or stay focused on writing content thus limiting the jobs I'm qualified for in today's changing market?