Greg - your point about specialization is indeed true, providing your time window is a narrow one and your area of specialty has sufficient headroom to support you and your competitors.

Generally over time if the specialty has enough draw, the market will produce competitors (barriers to entry aside) which will potentially erode your market share and revenue. And in the technology game, today's hot specialty - be it application, development platform, OS, etc. is tomorrow's orphaned investment. Which means that sooner or later, the thing you originally specialized in will no longer provide you with revenue opportunities.

So while I agree with your basic premise, I would also add that 2 specialties (one new and one old) are the minimum you should invest time, money and energy into, if you are intending to make independent contracting your way of life.