ok....I'll give you an extended answer
1) the expert recommendations you are reading are written from corporate planning point of view. In a corporate environment, secondary market research (i.e reports, statistics) plans, figures and forecasts are an essential part of corporate managers allocating budgets to specific project groups
in a corporate environment, it doesn't matter if the forecast is right, as long as there is a forecast, or the budgets don't get allocated
2) In a startup environment, the purpose of forecasting is different. You can't make up some numbers and show them to 3rd party so they can give you a budget.
Your forecasts are about getting prepayments and contracts for delivery from live prospects, so you can use these legal promises of future payment to recruit employees and partners. You can't get trade credit from a web designer because the Yankee group thinks your space will be worth $5.7 billion in 2008
What you need is a contract that says XYZ LLC will buy this stuff from us if you make the website (or something...)
That's the difference in practicalities of corporate and startup demand forecasting.
Your forecast horizon should only be as far as the next sale...not the value of network hardware in 2008 in North America, or how much money you're supposed to be making in 5 years time.
3) The discrepancy between my advice and expert advice is because you're not listening to the right experts. You're reading books written by corporate managers. If you go to
http://southerncrossventures.com/blogroll and ask for advice from actual entrepreneurs and venture capitalists who are building STARTUP COMPANIES everyday - they'll tell you the same thing as I am telling you.
4) Don't believe me? Go and ask
any of these guys on "how do I research demand?" and they'll tell you - "drop everything and go make some sales".