- If or when the next step comes you have to get another guy doing that work
- If you need to add even a tiny feature, you have to find a contractor/freelancer doing that
- If the design person hasn't even the slightest clue about basic front-end code, how is the collaboration going to work?
If you are something of a Jack of all trades, I'd guess you would be instantly more flexible on the job market. I've never been fond of locking myself into a single technology – I don't believe it's healthy neither for the mind nor the career. I also warmly recommend this article, 'In Defense of the Jack of All Trades', where the author brings up several honorable properties: endless curiosity, a love of learning, and a deep-seated need for control. Being a renaissance man on the web in 2011 is certainly possible.